BEIRUT (AFP) - US Secretary of State John Kerry met Lebanon's prime minister on Wednesday on an unannounced visit to Beirut with a focus on the humanitarian response to the war in neighbouring Syria.

In the first visit by a secretary of state to Beirut since 2009, Kerry was to announce US$290 million (S$360 million) in aid to boost UN efforts in Syria and neighbouring countries hosting war refugees, a US diplomatic source said.

He would also urge leaders in Beirut to address a political crisis by appointing a Lebanese president "as soon as possible".

Kerry's visit comes more than three years into a conflict that is raging in Syria, which has had major political, security and humanitarian consequences for tiny Lebanon.

The top US diplomat began by meeting Lebanese Prime Minister Tammam Salam, and he is also scheduled to hold talks with parliamentary speaker Nabih Berri and Maronite Patriarch Beshara Rai.

"The secretary is going to announce our next response to the UN High Commissioner for Refugees and UN system appeals," said the source, speaking on condition of anonymity.

Syria's war has created an unprecedented refugee crisis, pushing some three million people out of their country, including more than a million into Lebanon.

"Of the US$290 million that the secretary will announce, US$51 million of that will be for Lebanon. That will be the largest single chunk for the neighbours," said the source.

"The biggest chunk of anything we announce is the one that is going to Syria for displaced people and UN services there," he added.

The visit also comes during a protracted political crisis in Lebanon, which has been without a president since last month because of unbreachable divisions between pro- and anti-Damascus camps in the country.

The diplomatic source said Kerry's visit was in part aimed at showing "support for the government" of Salam, which has assumed executive powers since Michel Sleiman's mandate as president expired.

"We are seeing the prime minister. We would obviously be seeing the president but a president does not exist," said the source.

"This a time when we want to send a message that they should elect a president as soon as possible," he said, adding Lebanon should do that "without foreign influence or foreign interference."

Kerry would also urge support for the Lebanese army and security forces, the source added.