Nuclear weapon

The exchange appeared to raise the prospect of a new arms race between the two nuclear superpowers, which between them boast more than 14,000 nuclear warheads, the still deadly legacy of their four-decades long Cold War standoff.

But the comments by Putin, who is presiding over a project to restore Russia's lost global power and influence, and Trump, who will shortly become the US commander-in-chief, did not spell out exactly what each side is proposing or whether a major change of nuclear doctrine is in the offing.

Such sampling of soil, air or dust from equipment is usually done by the International Atomic Energy Agency's (IAEA) own experts. But IAEA chief Yukiya Amano confirmed that Iranians carried out that part of the probe at Parchin, where the agency suspects that explosive triggers for nuclear weapons might have been tested in the past.

Diplomats say Iran insisted on the compromise as a condition for any probe of Parchin.

Tehran and world powers struck a deal last month on Iran's contested nuclear program, and on Sunday Britain and Iran reopened their respective embassies after a break of several years.

Foreign Secretary Philip Hammond, who met Iranian President Hassan Rouhani in Tehran on Monday, said the two countries shared common ground despite a "deep legacy of distrust."