"All appropriate steps" must be taken to protect Americans from a possible al Qaeda attack, US President Barack Obama has said.

The US has issued a worldwide travel alert, citing an al Qaeda threat that also led to a decision to close US embassies around the Muslim world.

The alert came just hours before a car bomb exploded near the Indian consulate in the eastern Afghan city of Jalalabad on Saturday morning.

Eight children were killed and 21 other people were wounded in the attack, according to Sky sources. The youngsters were believed to have been attending a religious lesson in a nearby mosque.

The US State Department has warned American citizens of the potential for terrorism particularly in the Middle East and North Africa.

"The president is being updated on a potential threat occurring in or emanating from the Arabian Peninsula," said a White House statement.

"Current information suggests that al Qaeda and affiliated organisations continue to plan terrorist attacks both in the region and beyond, and that they may focus efforts to conduct attacks in the period between now and the end of August," the statement said.

The alert was posted a day after the US announced it would close diplomatic facilities on Sunday because of an unspecified threat.

State Department spokeswoman Marie Harf said the department acted out of an "abundance of caution" and that some missions may stay closed for longer than a day. Sunday is a business day in Muslim countries.

Representative Ed Royce, the Republican chairman of the House Foreign Affairs Committee, said on Friday the embassy threat was linked to al Qaeda and focused on the Middle East and Central Asia.

"We've had a series of threats," Mr Royce told reporters. "In this instance, we can take a step to better protect our personnel and, out of an abundance of caution, we should."

Representative Dutch Ruppersberger, the top Democrat on the House Intelligence Panel, described the threat as "not the regular chit chat" picked up from would-be militants on the internet or elsewhere.

The State Department issued a major warning last year informing American diplomatic facilities across the Muslim world about potential violence connected to the anniversary of the September 11 terrorist attacks.

In Benghazi, Libya, four Americans, including Ambassador Chris Stevens, were killed in an attack on the US consulate.

The deadly assault has prompted several calls for investigations from House Republicans who have accused the Obama administration of misleading Americans about the attack.

Sky News US political analyst Jon-Christopher Bua said the latest moves by the State Department are not based on the politics surrounding the Benghazi fallout.

"The latest closure announcement of the US embassies and today's alert for potential al Qaeda terrorist attacks seem to be driven by real information and not over cautious individuals with political motives," he said.

Friday's alert warned that al Qaeda or its allies may target US government or private American interests.

It cited dangers involved with public transportation systems and other prime sites for tourists, noting that previous terrorist attacks have centred on subway and rail networks as well as airplanes and boats.

Meanwhile, Britain announced it also will close its embassy in Yemen on Sunday and Monday amid "increased security concerns".