Washington has indicated it was prepared to talk directly with Iran. Talks involving all five permanent members of the U.N. Security Council plus Germany have made little headway, and several rounds of international sanctions have cut into Iran's oil sales and financial transactions.
In a defiant move last month ahead of new talks expected soon with the six powers, Iran announced plans to vastly step up its pace of enriching uranium, which can then be used to make reactor fuel and the fissile core of warheads.
Biden told the international security conference "there is still time, there is still space for diplomacy backed by pressure to succeed." He did not specify any timeframe.
He insisted "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: "When the Iranian leadership, the supreme leader, is serious."
The United States has long made clear it was prepared to meet directly with Iranian leaders, 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 are not just prepared to do it for the exercise," Biden told the Munich Security Conference.
Lavrov, whose country is a key player in the six-nation talks with Iran, said it was important to offer Iran clear incentives to resolve the nuclear standoff. "We have to convince Iran that it is not about the regime change," he said.
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. Iran's foreign minister, Ali Akbar Salehi, is to address the conference Sunday.
Though Russia and the United States have worked together on Iran, their differences over Syria were on display again at the conference in Munich, an annual gathering of top security officials.
In his speech, Biden stressed the conviction of the United States 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 "the opposition continues to grow stronger."
He later held separate meetings with Lavrov, international peace envoy Lakhdar Brahimi, and Khatib.
On the sidelines of the conference, Lavrov met for the first time with Khatib - who in December rejected a previous Russian invitation for talks, Russian news agency ITAR-Tass reported.
On Friday, Khatib said he was open to talks with Assad's government to "ease the pain of the Syrian people."
Lavrov welcomed that idea and said on his flight back to Moscow that Russia and the Syrian opposition planned to stay in regular contact, Russian state news agency RIA Novosti reported.
"This is an important step, given that the coalition was created on the platform of rejecting talks with the regime," he said. "But I think that realism will prevail. Naturally, this does not guarantee that dialogue will start, as the opposition does not have a negotiating team and there are many different groups to agree on a single delegation."
At the conference, Biden told an audience including Lavrov that 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."
But Lavrov fired back, saying, "There are a lot of question marks about the Western approaches to those developments" in the region, and questioning when it was "permissible to cooperate with regimes and when is it legitimate to argue for their removal."