Ansar al-Sharia, which is believed to be responsible for death of Chris Stevens, fled to remote Jebel Akhdar region in September

An Islamist militia suspected of the killing of America's Libya ambassador is being blockaded in a remote, forested region in the east of the country, but army commanders say they lack the firepower to capture the militiamen.

The Islamist Ansar al-Sharia fled to the hilly Jebel Akhdar – Green Mountains – region after protesters ejected its members from their bases in Benghazi and Derna in late September. Last week they ambushed a police checkpoint near the small town of Susah, killing four officers.

"They have 150 to 200 men and 17 vehicles, Toyotas and four-by-fours," said the army taskforce commander, Colonel Hamid Hassi. "These people are very dangerous."

Libyan commanders say the militiamen have returned to the area from where they mounted an Islamist resistance against the former dictator, Muammar Gaddafi, 20 years ago. "Gaddafi tried to fight these guys here, he had 30,000 soldiers, in 1992, and he could not catch them," Hassi said. "We need help from the United Nations or the Europeans."

Infantry units backed by pickup trucks carrying anti-aircraft guns have been set up in blocking positions on the coastal highway at Susah and Derna.

In between are 40 miles of some of Libya's wildest and most beautiful country, a landscape of forests and twisting valleys famed as the hideout of Saint Mark.

"You need good equipment to go in there," Hassi told the Guardian. "We asked the chief of staff to send us planes and helicopters, but we received nothing."

Drones have been heard in the night skies in the region, prompting speculation that the US is poised to take action, four weeks after ambassador Chris Stevens and three diplomats were killed in an attack on the US consulate in Benghazi.

Pressure for decisive action is mounting in Washington before a congressional hearing on Wednesday to examine the circumstances of the attack, and whether diplomats ignored prior warnings of jihadist violence.

The Republican presidential candidate, Mitt Romney, on Monday linked the consulate attack to al-Qaida, and questioned whether the Obama administration had the resolve to deal with it.

Libya's government is for the moment in disarray following the sacking by parliament of prime minister-elect Mustafa Abushagur on Sunday after his proposed cabinet was rejected.

Further west, Egypt and India have begun trying to evacuate nationals from the town of Beni Walid, a former Gaddafi stronghold now surrounded by government forces demanding the handover of men accused of murdering a prominent revolutionary. Tanks and artillery surround the town.

The murder of the four policemen has shocked the small town of Susah, home of the ancient ruins of the Greek settlement of Apollonia, and local people have dubbed the day of the killings "Black Wednesday". "This is a small place, people are very upset," said one resident, who asked not to be named for fear of reprisals.