The top White House anti-terror official John Brennan yesterday pressed Libyan leaders to hunt down those guilty of killing Ambassador Chris Stevens and three other Americans as controversy mounted in Washington.

Just four weeks ahead of the US presidential election, Barack Obama's administration faced mounting criticism from Republicans over the breach of security at the US consulate in Libya's second city Benghazi on September 11 that led to the killings.

Security was tight in the Libyan capital for Brennan's talks with national assembly chief Mohammed Megaryef, Libya's de facto head of state, and other senior officials.

Brennan set out to Megaryef "specific additional steps Libya can take to better assist the US in ensuring that the perpetrators are brought to justice," the White House said.

Brennan also urged Libya to "take full and

timely advantage of specific offers of assistance from the United States and other international partners."

The envoy's visit came with political temperatures rising in Washington as senior State Department officials gave evidence to a congressional hearing into the deadly consulate attack.

In a dramatic new account, two State Department officials Tuesday described a relentless attack in which dozens of armed men invaded the US consulate in Benghazi, setting it on fire and hunting through the building for staff.

There had been no warning that an attack was planned, and in the hours before the streets outside the compound had been calm, they said on a conference call with reporters, asking to remain anonymous.

The new account contradicts initial reports by State Department officials which said it was a "spontaneous" attack sparked by a protest against an anti-Islam film.