Nicholas Burns, the State Department's under secretary for political affairs, said in congressional testimony that the United States was grateful for the "patient, principled diplomacy" by the "European Union Three" -- France, Germany and the United Kingdom -- in negotiating with Iran, which has announced it will resume uranium conversion work, in violation of the nuclear Non-Proliferation Treaty (NPT.)

But Burns signalled US scepticism about whether a mutually acceptable agreement can be reached, and expressed doubts about Tehran's motives.

"The bar for Iran must be set very high because of its history of deception" in years of concealing its nuclear program, Burns told members of the Senate Foreign Relations Committee.

"We see no sign Iran has made the necessary strategic decision to abandon its nuclear weapons program," Burns said.

"Iran's repeated brinksmanship in its negotiations with the European Union Three ... is part of Iran's continuing effort to divide the international community, weaken our resolve and avoid adhering to its international obligations," he said.

Even if the parties ultimately reach a resolution, Burns said Washington was unlikely to accept any such agreement on face value.

"If we don't trust, we really must verify," Burns said.

The senior US diplomat made his remarks as the European Union ratcheted up pressure on Tehran, ahead of crisis talks between the sides next week.