British and Nato military commanders are planning what they hope will be a final onslaught on Colonel Gaddafi's forces to put an end to all resistance from troops loyal to the Libyan leader.

Heavy fighting raged around Gaddafi's Bab al-Aziziya compound, in Tripoli, on Tuesday afternoon as rebels rained artillery rounds, mortar shells and missiles on loyalist positions.

Columns of grey smoke billowed over the Libyan capital as witnesses reported a buildup of rebel troops and vehicles to the east of the compound.

Large convoys of rebel vehicles raced through deserted streets in an apparent show of strength after Gaddafi's son Saif al-Islam claimed the government had "broken the backbone" of the opposition.

After being caught by surprise by the speed of the rebel advance on Tripoli, Nato chiefs have ordered what defence officials described a "tactical pause" in the bombing campaign.

But the pause will not last long, and the bombing of what strategic targets are left in Tripoli will resume, possibly as early as Tuesday night, alliance officials said.

The Guardian has learned that a number of serving British special forces soldiers, as well as ex-SAS troopers, are advising rebel forces, although their presence is officially denied.

Two thousand rebel reinforcements arrived in Tripoli on Monday night after breaking through government lines near Zlitan, according to Guma al-Gamaty, the London representative of the rebel National Transitional Council. "They should make a difference," he said.

More rebel fighters arrived by boat, and a separate convoy of jeeps and artillery was heading west from Misrata, according to rebels in the eastern city, which had been besieged by government forces for five months.

The sudden advance on the capital suggests co-ordination between the rebels and Nato planners is not as effective as had been widely assumed.

On Tuesday, Nato commanders were analysing photographic and signals intelligence provided by spy planes looking at what defence chiefs call "patterns of life" – movements of people and vehicles in and around Gaddafi's compound.

British, Danish and Norwegian aircraft have been particularly active in striking targets in Triploi. RAF jets have attacked the compound with 500lb Paveway bombs, but they have so far been directed at its perimeter walls and control towers.

The decision facing Nato commanders on Tuesday was whether the compound's core and underground tunnels could be regarded as legitimate targets and weighing up the risks involved, notably to the lives of civilians and rebels. British defence chiefs are also aware of the dangers of being seen to be sanctioning assassination.

Nato planes can more easily spot groups of Gaddafi forces ambushing rebel convoys on the streets of Tripoli, but defence officials say bombing them from the air would be far too risky.

Pilots are continuing to seek targets that are more clearly defined as military, including command and control facilities, radar and surface-to-air missiles which are still being operated by troops loyal to Gaddafi, the latest strike figures put out by Nato indicate.

British aircraft are seeking what pilots call "dynamic" targets – targets seen by chance – as well as "deliberate" planned targets.

The Guardian has previously reported the presence of former British special forces troops, now employed by private security companies and funded by a number of sources, including Qatar. They have been joined by a number of serving SAS soldiers.

They have been acting as forward air controllers – directing pilots to targets – and communicating with Nato operational commanders. They have also been advising rebels on tactics, a task they have not found easy.

Britain's international development secretary, Andrew Mitchell, said there would be a "bumpy ride" over the coming days.

"There was a lot of confusion. There are quite long lines of communication involved," he told the BBC. "It's inevitable in this situation, with the warfare going on as it is, that there will be some confusion."