The United States moved on Thursday to reduce its diplomatic presence in Syria following this week's attack on its embassy in Damascus by offering free flights out of the country to non-essential diplomats and family members on a voluntary basis.

The State Department announced the move in a travel warning that alerted U.S. citizens to "increased security concerns in Syria" following the attack on Tuesday.

Four men shouting Islamic slogans tried to blow up the embassy in Damascus but their plot was foiled after Syrian guards killed three of them in a shootout.