By Carol Giacomo, Diplomatic Correspondent

WASHINGTON (Reuters) - Faced with an increasingly hard line from Iran, the United States and Europe have stepped up planning for tougher diplomatic action should Tehran follow through on threats to resume critical nuclear activities, according to U.S. officials and European diplomats.

The U.S. and its European allies are seeking agreement among themselves on precisely when Iran's nuclear program will have progressed to the point that the matter should be taken to the U.N. Security Council and what kinds of sanctions might be pursued there, the officials and diplomats said.

Tehran insists it only aims to produce civilian nuclear energy. Allies say the program is to produce weapons.

Russia, which is building Iran's nuclear power plant at Bushehr in southern Iran, remains a serious impediment. The United States fears that weapons grade plutonium could be extracted from the Bushehr reactor once it goes on line.

The United States and major European nations -- Britain, France and Germany -- have long threatened to bring the issue to the U.N. Security Council for possible sanctions.

But negotiations appear at an impasse and new Iranian President Mahmoud Ahmadinejad has alarmed the world with aggressive calls for Israel to be "wiped off the map."

"Increasingly, we feel the Iranians are just not interested in any sort of privately negotiated solution to this problem, that what they are interested in is a political confrontation over it," one European diplomat told Reuters.

Under the nuclear Non-proliferation Treaty, which Iran signed, member states are guaranteed the right to develop a full nuclear fuel cycle but are banned from making weapons.

The Bush administration is under growing pressure from Congress and pro-Israel groups to soften its stance toward Tehran. They want the nuclear issue referred to the U.N. Security Council, where sanctions could be imposed.

U.S. Undersecretary of State Robert Joseph, who oversees nonproliferation issues, was in Europe this week for meetings that included discussions on Iran.

U.S. and European experts are to meet Iran next week to see if negotiations can resume, but the outlook is pessimistic.

"I think there are a lot of different pieces moving toward an interesting point on Iran, especially the nuclear piece," a U.S. official said.

A pro-Israel advocate said administration officials "are considering harder approaches. Things are moving on a faster track."

A second European diplomat said while there was a U.S. trend to "toughen the position" on Iran, some Europeans preferred to keeping trying to draw Russia into a unified position.

Efforts to halt Iran's nuclear program would suffer if the issue was moved to the Security Council and the council was too divided to take action, some analysts said.

U.S. officials say if the Security Council discussed Iran's nuclear program, sanctions would not be imposed immediately, while the council tried other diplomatic pressures.

WHERE IS THE 'RED LINE'?

Also under discussion is what the United States and other states would consider their "red line" -- the point at which Iran has crossed into a dangerous activity that cannot be tolerated.

"We cannot achieve anything until we are certain we see things the same way," the second European diplomat said.

Iran froze work at its Isfahan nuclear facility in late 2004 under a deal with Britain, France and Germany but resumed uranium conversion in August 2005.

Tehran has threatened to go further and begin uranium enrichment, the most sensitive part of the nuclear cycle. The United States, Britain, France and Germany generally agree any further steps would be unacceptable but Russia is more lenient, officials said.