At least two Western embassies received warnings of new security threats in Ankara, and Serbia warned its citizens not to travel to the country two weeks after a bomb attack in central Istanbul.

Three Western European embassies and a major international organization told Reuters that Turkish authorities had warned them of potential threats, asking them not to name them because of the sensitivity of the issue.

A major organization, in an email to employees seen by Reuters, stressed risks in shopping malls, bus terminals and airports because of the possibility of retaliatory attacks by militants.

Turkey's defense and interior ministries declined to comment on the warnings.

Meanwhile, Serbia's Tanjug news agency quoted the country's foreign minister, Ivica Dacic as saying that citizens should avoid traveling to Turkey and especially Ankara in the coming weeks because of possible attacks.

"I want to warn the citizens that according to information from the security agencies terrorist attacks can be expected in the next few weeks, and in this case this information refers to the capital of Turkey, Ankara," Dacic said in a statement to Tanjug.

The warning may also apply to other parts of Turkey, Dacic said, and warned Serbian citizens not to travel to the country without an urgent need. "If our citizens are already there, they should avoid crowded places such as pedestrian areas, subways, bus and train stations and other such places," he said.