MUNICH, Germany - The United States is prepared to hold direct talks with Iran when the Iranian leadership is "serious", US Vice President Joe Biden said on Saturday.

Speaking at the Munich Security Conference Saturday, Biden said that "we have made it clear at the outset that we would be prepared to meet bilaterally with the Iranian leadership," adding that the US is "not just prepare to do it for the exercise."

The US vice president arrived in Germany on Friday for the conference. In an interview with German Sueddeutsche Zeitung daily on Friday, Biden urged Iran to return to negotiation table, warning that the time window for "successful diplomacy" will be kept open, "however, not unlimitedly."

On Thursday, the US Government had slammed Iran's plan to install new centrifuges at its main enrichment facility in Natanz, calling it "a further escalation" over its disputed nuclear program.

Iran informed the plan to the International Atomic Energy Agency (IAEA) last week as it was engaged in discussions about resuming nuclear talks with the so-called "P5+1", referring to the five permanent members of the United Nations Security Council -- Britain, China, France, Russia, the United States -- plus Germany.

The UN Security Council and Western powers have imposed crippling sanctions on Iran for years over its refusal to stop its uranium enrichment activities, while Iran insists on the peaceful nature of the program.

"That pressure will continue, and it will increase, as long as Tehran refuses to live up to its international obligations with regards to its nuclear program," said White House spokesman Jay Carney on Thursday.

"There are still time, there are still space for diplomacy backed by pressure to succeed," said Biden in Munich on Saturday, adding that "the ball is in the government of iran's court."