US Secretary of State John Kerry on Sunday said the window for diplomacy with Iran was "cracking open" following talks in London with EU foreign policy chief Catherine Ashton.

Kerry met Ashton in London for talks covering Syria and the forthcoming meeting between Iran and world powers, according to a State Department official.

The two-day talks kick off in Geneva on Tuesday amid raised hopes of a less hardline approach from the Islamic republic's new president, Hassan Rouhani, a reputed moderate who took office in August.

"Right now, the window for diplomacy is cracking open," Kerry told the American Israel Public Affairs Committee summit in Washington via satellite from London.

"But I want you to know that our eyes are open, too," he added. "When we say that Iran must live up to its international responsibilities on its nuclear programme, we mean it.

"When President Obama says that he will not allow a nuclear-armed Iran, he means what he says. I believe firmly that no deal is better than a bad deal."

Iranian Foreign Minister Mohammad Javad Zarif will negotiate with Ashton and the so-called P5+1 group of the United States, Britain, China, France and Russia plus Germany.

Kerry flew into the British capital earlier Sunday from Kabul, on his way back to Washington following a 10-day trip around Asia.

"They had a discussion about ongoing global issues of mutual concern," the State Department official said after the Kerry-Ashton talks.

These included "the recent breakthrough on BSA (bilateral security agreement) negotiations which the EU will be closely engaged with, the upcoming Geneva conference with the P5+1 on Iran, the situation on the ground in Egypt, as well as efforts to plan a conference in Geneva on ending the civil war in Syria through a political transition," he said.

"They also discussed the ongoing direct negotiations between the Israelis and the Palestinians and agreed to increase joint efforts on the Palestinian economic initiative."

During his address to AIPAC, Kerry pledged US support for Israel, but repeated that a two-state plan was the "only solution" to bring peace.

There are increased hopes of a diplomatic resolution to the decade-long standoff over Iran's nuclear programme following Rouhani's pledges to engage with the major powers in order to secure a lifting of sanctions.

Iran's supreme leader Ayatollah Ali Khamenei last week backed Rouhani's overtures.

The US is sending Wendy Sherman, the under secretary of state for political affairs, to the Geneva talks.

In London on Monday morning, Kerry was to meet Lakhdar Brahimi, the United Nations-Arab League envoy on Syria. They were also due to speak to press, the State Department said.

Their talks will likely revolve around preparations for long-stalled so-called "Geneva-2" peace talks aimed at finding a political solution to the conflict in Syria.

The United States and Russia have been trying to bring together members of President Bashar al-Assad's government and rebel representatives for a Geneva conference following the failure of a first round of talks in June.