In this image taken from video obtained from the Erbin Ciity, which has been authenticated based on its contents and other AP reporting, U.N. investigation team with blue helmets speak with Free Syrian Army fighters in the Damascus countryside of Zamalka, Syria, Thursday, Aug. 29, 2013. Syria's state news agency quoted President Bashar Assad as saying Syria will defend itself against any aggression. AP Photo/Erbin City via AP video Britain's Prime Minister David Cameron leaves 10 Downing Street in London, to be driven to the Houses of Parliament for a debate and vote on Syria, Thursday, Aug. 29, 2013. Britain's opposition Labour Party has indicated it may not support even a watered down version of a government resolution on Syria. UN vehicles leave the Four Season Hotel in Damascus, Syria, Thursday, Aug. 29, 2013. DAMASCUS, Syria (AP) - Syrian President Bashar Assad said Thursday that his country "will defend itself against any aggression," signaling defiance to mounting Western warnings of a possible punitive strike over a suspected poison gas attack blamed on his regime. U.N. chemical weapons inspectors toured stricken rebel-held areas near the Syrian capital of Damascus for a third day Thursday. Amateur video posted online showed U.N. inspectors wearing gas masks walking through a damaged building. One inspector scooped pulverized debris from the ground, placed it in a glass container and wrapped the container in a plastic bag. The inspectors' departure from Syria on Saturday could clear the path for possible military action against the Assad regime. U.N. Secretary-General Ban Ki-moon on Thursday urged Western powers to hold off on any decisions until his experts can present their findings to U.N. member states and the Security Council. The suspected chemical weapons attacks took place Aug. 21 in suburbs east and west of Damascus. The humanitarian group Doctors Without Borders has said the strikes killed 355 people. President Barack Obama said he has not decided how the U.S. will respond. However, he signaled Wednesday that the U.S. is moving toward a punitive strike, saying he has "concluded" that Assad's regime is behind the attacks and that there "need to be international consequences." The U.S. has not presented proof in public. The Syrian regime has denied a role in the attacks, alleging instead, without presenting evidence, that anti-government rebels carried them out to frame Assad. The Syrian president struck a tough tone Thursday. His comments, from a meeting with a delegation from Yemen, were reported by the state news agency SANA. "Threats to launch a direct aggression against Syria will make it more adherent to its well-established principles and sovereign decisions stemming from the will of its people, and Syria will defend itself against any aggression," Assad said . It's not clear if Assad would retaliate for any Western strikes or try to ride them out in hopes of minimizing the threat to his continued rule over parts of Syria. The U.S. has said regime change it not its objective. Meanwhile, the possibility of a Western strike and a Syrian response has sparked anxiety among civilians in neighboring countries. Israelis stood in long lines Thursday for government-issue gas masks. Turkey's government crisis management center said officials had designated bunkers at seven areas along the border. And Lebanon's foreign minister, Adnan Mansour, warned that international military action against Syria would pose a "serious threat" to the security and stability of the region, particularly in Lebanon. Meanwhile, both Obama and British Prime Minister David Cameron were trying to shore up domestic political support Thursday for possible military action. The Obama administration was planning a teleconference briefing Thursday on Syria for leaders of the House and Senate and national security committees, U.S. officials and congressional aides said. Cameron convened Parliament for an emergency meeting to vote on possible international action against Syria. Ahead of the session, the British government released documents meant to bolster the case that chemical weapons were used by Syria, including an intelligence assessment that said regime involvement was "highly likely." The government also said legal conditions have been met for taking action against Syria. Earlier, Cameron had promised lawmakers he would not go to war until the U.N. weapons team has had a chance to report its findings. The speaker of the Syrian parliament, Jihad Allaham, sent a letter to his British counterpart, urging British lawmakers not to endorse military action. In Vienna, Ban said he spoke to Obama a day earlier about ways to expedite the U.N. investigation. Ban said the U.N. team is set to leave Syria on Saturday, and suggested that Western powers hold off on any decisions until the inspectors have presented their findings. Ban said he told Obama on Wednesday that the U.N. investigators "should be allowed to continue their work as mandated by the member states and I told him that we will surely share our information and our analysis." "Diplomacy should be given a chance, and peace given a chance," Ban said. It's important that all the differences of opinions should be resolved through peaceful means and through dialogue. The U.N. inspectors toured the eastern Damascus suburb of Zamalka on Thursday, according to anti-regime activists and amateur video. Other videos showed the convoys of U.N. vehicles, accompanied by armed rebels in pickup trucks. The videos were consistent with AP reporting from the area. The U.N. team did not issue a statement about its plans Thursday. On two previous tours this week, the inspectors visited a western suburb of the city as well as Zamalka where they took biological samples from suspected victims. Ban has said the samples would be analyzed and presented to the U.N. Security Council. In countries neighboring Syria, governments began taking precautions against possible Syrian retaliation. Mansour, the Lebanese foreign minister, warned in an interview with The Associated Press that a Western military strike would escalate tensions in Lebanon and dramatically increase the number of Syrian refugees. Lebanon is a tiny country that shares a porous border with Syria, and has seen cross-border shelling, sectarian clashes and car bombings in recent months related to the civil war raging next door. There are concerns that U.S. military action in Syria may trigger another wave of refugees fleeing across the border into Lebanon - the country of 4.5 million already is already host to nearly 1 million Syrian refugees - and trigger violence across the country. Israel, meanwhile, has called up reservists and deployed missile defense batteries in preparation for a possible Syrian response to an American attack. In Turkey, the government's crisis management center said on Twitter that a team of 100 chemical weapons experts were sent to the border area, which was being screened for any signs of chemical attacks. Turkey is Assad's strongest critic and has backed Syria's opposition and rebels. The country said this week it would take part in any international coalition that would move against the Syrian government. Associated Press writers Yasmine Saker in Beirut, Tia Goldenberg in Jerusalem, Gregory Katz in London and George Jahn in Vienna contributed reporting.