Hillary Clinton, the US secretary of state, will travel to Turkey on Saturday for talks over the worsening crisis in Syria, according to a state department spokeswoman.

"Clinton goes to Istanbul for bilateral consultations with the Turkish government on Syria as well as to cover other timely issues," the spokeswoman said on Sunday in the Malawian capital, Lilongwe.

Clinton, who arrived in Malawi on the latest leg of her Africa tour, has added trips to Nigeria and Benin before travelling to Istanbul, the spokeswoman said.

The conflict in Syria has claimed at least 18,000 lives since the uprising against the regime of President Bashar al-Assad began in March last year, according to opposition activists.

Barack Obama, the US president, and Recep Tayyip Erdogan, Turkey's prime minister, agreed at the end of July to accelerate political change in Syria, including Assad leaving power.

Cross-border attacks

Ankara has become a champion of the uprising against Assad's regime and given refuge to large numbers of army defectors, who have formed a rebel army - called the Free Syrian Army - as well as tens of thousands of civilian refugees.

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Violence continues and Syria's second city Aleppo has been the scene of fierce fighting since the rebels launched a major offensive on July 20 from rear bases across the nearby border with Turkey.

Turkey moved troops to its border with Syria in June after a Turkish Phantom F-4 jet was shot down by Syria over the eastern Mediterranean in what Erdogan called a "heinous attack" over international waters.

A month later Turkey deployed armoured combat vehicles, troops, ammunition and missile to strengthen the fortified border.

Obama has signed a secret order authorising US support for rebels seeking to depose Assad and his government, according to the Reuters news agency.

Obama's order, approved earlier this year and known as an intelligence "finding", broadly permits the CIA and other US agencies to provide aid that could help the rebels dislodge Assad from power.

Separately from the president's secret order, the Obama administration has stated publicly that it is providing some backing for Assad's opponents.

Source: Agencies