Nuclear Negotiators Meet in Bid to Revive Iran Talks The top nuclear negotiators for Iran and the big powers met face to face for the first time in nearly three months on Tuesday, part of an effort to revive the stalled talks over Iran's disputed uranium enrichment activities as new military tensions roil the Middle East. The meeting between the negotiators, Catherine Ashton, the European Union's top foreign policy official, and Saeed Jalili, the secretary of Iran's Supreme National Security Council, was held privately in Istanbul and was not considered a formal negotiation. Michael Mann, Ms. Ashton's spokesman, said in an e-mailed statement that it was "a useful and constructive meeting and an important opportunity to stress once again to Iran the urgent need to make progress." The last time Ms. Ashton and Mr. Jalili met was June 18-20 in Moscow, where teams for Iran and the P5-plus-1 countries - the five permanent members of the United Nations Security Council plus Germany - walked away in frustration. A series of lower-level contacts since then led to the Istanbul meeting, which came against a backdrop of tightening Western economic sanctions on Iran, new disclosures of sabotage at Iran's uranium enrichment sites and shrill exchanges between Iran and Israel over the possibility that Israel may attack Iran. Iran has insisted that it is enriching uranium for peaceful uses and has rejected Security Council demands that it halt the activity until questions about the possible weaponization of its nuclear fuel stockpile are answered. Last week the International Atomic Energy Agency, the monitoring arm of the United Nations, rebuked Iran for refusing to grant inspectors unlimited access to sites where they believe suspicious activity may have occurred. Iran's most senior atomic energy official responded on Monday by asserting that the I.A.E.A. had been compromised by "terrorists and saboteurs," and he disclosed that power lines to its two main uranium enrichment sites had been hit by explosions. The official, Fereydoon Abbasi, did not say whether the sites had been damaged, but his assertions raised the possibility that Israel, the United States or some other groups had carried out another attack in a clandestine effort to subvert the enrichment program short of a military attack. The I.A.E.A. did not comment on Mr. Abbasi's assertions, but in a statement on Tuesday it said that the agency's director general, Yukiya Amano, had met with Mr. Abbasi and told him "it is essential for Iran to extend its full cooperation to the agency." In a rising show of military force meant as a signal to Iran, the United States is leading a 30-nation naval mine-clearing exercise in the Persian Gulf in 12 days of maneuvers that began on Monday. Iran responded on Tuesday by deploying what the state-run news media called a homemade missile destroyer, a newly refurbished submarine and a new underground missile launchpad in the northern city of Noshahr. Western powers have said negotiations are the preferred solution to the nuclear issue. Britain added pressure on Tuesday, with its foreign secretary, William Hague, telling lawmakers in London that although the sanctions were having a major impact, "we will be intensifying those sanctions in the coming weeks and months in the absence of successful negotiations." In the United States, United Against Nuclear Iran, an advocacy group that helped pressure a global banking communications network to expel sanctioned Iranian banks, said it had undertaken a new effort to force the authorities who assign international Internet addresses to block sanctioned Iranian entities and persons from access to the Web. In letters sent to the Internet Corporation for Assigned Names and Numbers, known as ICANN, based in Los Angeles, and the Réseaux IP Européens Network Coordination Centre, known as RIPE NCC, based in Amsterdam, the advocacy group's president, Mark D. Wallace, said they may be in violation of Iran sanctions and that by disconnecting Internet access, "the dictatorial regime of Iran would be severely impeded in pursuing its illegal and amoral activities." ICANN officials did not respond to requests for comment. RIPE NCC, in a statement posted on its Web site, said that it did not believe it had violated sanctions "but we will investigate in cases where new information is provided to us and we will ensure that changing circumstances do not place the RIPE NCC in violation of sanctions."