U.S.: Assad backers must justify carnage PARIS, April 19 (UPI) -- Syrian regime backers must justify how they can let the bloodshed go on, U.S. Secretary of State Hillary Clinton said before meeting with counterparts Thursday. "The burden has shifted," Clinton told reporters in Brussels Wednesday before flying to Paris for a foreign ministers meeting to boost pressure on Syrian President Bashar Assad's regime, a week after a battered cease-fire began. The onus has moved "not only to the Assad regime, but to those who support it, to be forced to explain why, after time and time again stating that they will end the violence, the violence continues," Clinton said. "The guns of the Assad regime are once again firing in Homs, Idlib and elsewhere, and Syrians continue to die," she said. Syrian security forces opened fire Wednesday on anti-regime demonstrators surrounding the four-wheel-drive vehicles of U.N. observers sent to monitor the cease-fire, activist groups said and amateur videos indicated. The barrage of bullets -- in the suburban city of Arbeen near Damascus, the capital -- sent the observers' vehicles racing away, sirens blaring, and protesters scattering, running for cover, the amateur videos posted online showed. "They are firing on demonstrators while the observers are visiting," a narrator in one video said. A video taken before the shooting showed a pink handwritten sign taped to the back of one of the U.N. vehicles saying, "The killer keeps killing, the observers keep observing and the people keep rebelling." Security forces and soldiers loyal to the Assad regime killed 46 people Wednesday in Homs, Idlib, Daraa, Hama, Deir Ezzor, Aleppo, Damascus and the Damascus suburbs, the opposition Local Coordination Committees of Syria said. The state-run Syrian Arab News Agency said an "armed terrorist group" detonated an explosive device in the Idlib province that killed six Syrian law-enforcement officers. It said a sniper killed a police officer in Daraa. The Assad regime says it has a right to respond to "terrorist" attacks, despite agreeing to a six-point peace plan brokered by U.N.-Arab League envoy Kofi Annan. The plan calls for the opposition to end the violence as well. "We are at a crucial turning point," Clinton told reporters. Either we succeed in pushing forward with Kofi Annan's plan in accordance with the Security Council direction, with the help of monitors steadily broadening and deepening a zone of non-conflict and peace, or we see Assad squandering his last chance before additional measures have to be considered. Clinton spoke with several ministers "about the need to tighten sanctions, tighten pressure on the regime, on those who support the regime," she said, "And we also are going to continue pressing for a political solution, which remains the goal of Kofi Annan's plan and the understandable goal of anyone who wants to see a peaceful transition occur in Syria." Clinto pointed out the deadly conflict was "taking place right on NATO's border" and spilled into NATO member Turkey for the first time April 9 when Syrian forces fired across the border near a refugee camp, killing two Syrian refugees and wounding at least 23 other people, including a Turkish police officer. "Our NATO ally, Turkey, has already suffered the effects of not only the influx of refugees that it is very generously housing, but also having two people killed on their side of the border because of Syrian artillery," Clinton said. The Obama administration "will judge the Assad regime by their actions, not their words," she said, adding she looked forward to Thursday's foreign ministers meeting, which was to include the Turkish foreign minister. Annan was to report to the Security Council Thursday on the progress of the observers' mission.