U.S. Secretary of State Hillary Clinton had held talks with her counterparts from a number of Western countries on the crisis in Syria, a U.S. official said on Tuesday.

Clinton had a conference call for more than an hour on Monday with British Foreign Secretary William Hague, French Foreign Minister Laurent Fabius, German Foreign Minister Guido Westerwelle and Turkish Foreign Minister Ahmet Davutoglu on the issue, State Department spokeswoman Victoria Nuland told reporters at a briefing.

They discussed the support for the Syrian opposition, the refugee problem and the preparation for the post-Assad period, according to the spokeswoman.

"The conversation very much focused on ensuring that we're all pulling in the same direction, that we're all sharing information, that we are thinking about the division of labor so we're covering all the bases without tripping over each other," Nuland said.

Clinton and Davutoglu held similar talks in Istanbul on Saturday. Following the talks, Clinton vowed to continue to support the Syrian opposition and announced additional American humanitarian aid to Syrian people.

The Western powers have been demanding Assad's ouster and used both diplomatic and economic sanctions to exert pressure on the Syrian government. Together with some Arab countries, they have also agreed to provide assistance to the Syrian opposition.

Russia and China insist any proposed resolution on Syria should be balanced and stipulate binding articles for both the government and opposition. The two UN Security Council permanent members have also opposed any outside military intervention against Syria.