MUNICH: The United States is prepared to hold direct talks with Iran in the standoff over its nuclear ambitions, Vice President Joe Biden said Saturday – but he insisted that Tehran must show it is serious and Washington won't engage in such talks “just for the exercise.”

Washington has indicated in the past that it's prepared to talk directly with Iran, and talks involving all five permanent members of the UN Security Council plus Germany have made little headway.

Several rounds of international sanctions have cut into Iran's oil sales and financial transactions.

Last month, Iran, in a defiant move ahead of a new round of talks expected soon with the six powers, announced plans to vastly increase its pace of uranium enrichment. That can be used to make both reactor fuel and the fissile core of warheads.

Biden told an international security conference that “there is still time, there is still space for diplomacy backed by pressure to succeed.” He did not specify any timeframe.

He also insisted that “the ball is in the government of Iran's court” to show that it's negotiating in good faith.

Asked when Washington might hold direct talks with Tehran, Biden replied that “when the Iranian leadership, the supreme leader (Ayatollah Ali Khamenei), is serious.”

The US has long made clear that it is prepared to meet directly with the Iranian leadership, he added – “that offer stands but it must be real and tangible and there has to be an agenda that they're prepared to speak to.”

“We're not prepared to do it just for the exercise,” Biden told the Munich Security Conference.

Iran insists it does not want nuclear arms and argues it has a right to enrich uranium for a civilian nuclear power program, but suspicion persists that the real aim is nuclear weapons.

The Islamic Republic hid much of its nuclear program until it was revealed from the outside more than a decade ago. And defying UN Security Council demands that it halt uranium enrichment, Iran has instead expanded it.

“Iran should not wait any longer to take up the willingness Vice President Biden has stressed to hold substantial negotiations on its nuclear program,” said Foreign Minister Guido Westerwelle of Germany, whose country has been one of those trying to resolve the issue. He added that 2013 would be “decisive” for hopes of a diplomatic solution.

“From our point of view, announcing an accelerated expansion of uranium enrichment in Iran is the wrong signal,” Westerwelle said. Biden underlined that “our policy is not containment – it is to prevent Iran acquiring a nuclear weapon.”

The conference – an annual gathering of top security officials – also gave Biden an opportunity to address the civil war in Syria.

He planned to hold separate meetings with Russian Foreign Minister Sergey Lavrov, international peace envoy Lakhdar Brahimi, and Syria's top opposition leader, Moaz al-Khatib.

Russia is a longtime ally of Syrian President Bashar Assad.    Biden stressed the conviction of the US and many others that “President Assad – a tyrant hell-bent on clinging to power – is no longer fit to lead the Syrian people and he must go.”

He said that “the opposition continues to grow stronger.”

Despite differences, “we can all agree on the increasingly deep plight of the Syrian people and the responsibility of the international community to address that plight,” he told an audience that included Lavrov.

But Lavrov fired back that ''there are a lot of question marks about the Western approaches to those developments,'' in the region, asking whether supporting anti-government protesters justified terrorists, and questioning when it is ''permissible to cooperate with regimes and when is it legitimate to argue for their removal.''

''We are all interested in the stability of the Mideast and the African continent,'' and for governments to be democratic and peaceful, Lavrov said. ''If we agree on these common objectives we could probably agree on some transparent and common rules for all actors to follow.''

Biden also sketched out the Obama administration's foreign policy priorities for its second term, taking care to reassure Europeans that its ''pivot'' to Asia and the Pacific following a decade-long entanglement in Iraq and Afghanistan won't come at the expense of transitional trans-Atlantic ties.

''This engagement does not come at Europe's expense,'' he said. ''It's profoundly in Europe's interest for America to engage more broadly with the world.''