Iranian Foreign Minister Manouchehr Mottaki warned the United Kingdom not to cross "red lines," Reuters reported Saturday. "I told him that 'You have used a word, and I think it is a forbidden word ... Don't pass those red lines. Be careful about that'," said Mottaki, relating a conversation with his British counterpart David Miliband. Mottaki did not specify what those "red lines" referred to. The comments were made at a news conference which was broadcast and translated by Iran's Press TV. According to the Reuters report, the Iranian foreign minister had received word from Miliband that world powers planned to "write a letter" to Iran. Responding to this news, Mottaki said that Iran had "not received any letter in this connection," according to the semi-official ISNA news agency. Also Saturday, six world powers negotiating with Iran over its nuclear program, proposed that Teheran ceases uranium enrichment only for the duration of talks with the West, Russian Foreign Minister Sergei Lavrov said. Lavrov said that this apparent change in policy did not come without conditions and that in order to hold talks with the West, Teheran must cooperate. "Our first condition is a freeze, a suspension of uranium enrichment," he said. "The view of the six world powers is that Iran must keep enrichment suspended only while the negotiations are being held." Lavrov was speaking in London after the world powers - United States, Britain, France, Russia, China and Germany - agreed to try again to lure Iran to the nuclear bargaining table with a repackaged set of incentives to accompany the threat of UN sanctions. Diplomats said the offer contained no major new enticements, but was meant to remind the clerical regime that talking is still an option. The central terms of a 2006 compromise stand: Iran could trade away worrisome elements of its nuclear program for economic and political incentives and the possibility of a better relationship with arch-rival Washington. Iran turned down that invitation, saying it came with insulting strings attached, and Western diplomats were hard-pressed to say why the response would be any different today. The United Nations Security Council has imposed three sets of mild financial and other sanctions on Iran as a cost of spurning the offer. The strategy session on Iran was part of diplomatic meetings on the Mideast and Kosovo attended by UN Secretary-General Ban Ki-moon, US Secretary of State Condoleezza Rice and other international leaders. Details of the amended offer being made by the United States, Britain, France, Russia, China and Germany were not revealed, and the precise text is still a work in progress. "We very much hope that they will recognize the seriousness and the sincerity with which we've approached this issue," Miliband said following a meeting of senior diplomats from all six nations. Miliband did not give Teheran a deadline to respond or say what the group would do if Iran rejects the package again. As a condition of winning approval for the latest punishments last month, the United States agreed to revisit the list of rewards. Rice has said she doubts Iran is interested, no matter what. "I don't see any evidence that the Iranians appear to be interested in that track, and that doesn't leave us with any options" other than sanctions, Rice told reporters Thursday. European diplomats had said they hoped to sweeten the offer a bit, but Miliband did not characterize it that way. Rather, he called the new offer an update. Western diplomatic officials later said the offer restates but does not improve the original offer. They spoke on condition of anonymity to describe the offer not yet presented. The incentives offered in 2006 included an offer by the United States to provide Iran with peaceful nuclear technology, lift some sanctions and conduct direct negotiations with Teheran.