The military operation over Libya is having a "very real effect", the Ministry of Defence says.
A spokesman said Col Muammar Gaddafi's assault on Benghazi had been "stopped in its tracks".
British Typhoon and Tornado aircraft have continued to patrol the no-fly zone over Libya, while fighting continues on the ground.
Foreign Secretary William Hague said the operation would continue "as long as necessary" to achieve its aims.
Coalition operations in Libya are into a fourth day.
Chancellor George Osborne says the cost of UK involvement should be modest compared with its role in Afghanistan.
He told the Commons the estimates were "in the order of the tens of millions, not the hundreds of millions of pounds".
The Chief of the Defence Staff's spokesman, Maj Gen John Lorimer, highlighted the impact of the operation over Benghazi, the main stronghold of the rebels.
"Col Gaddafi vowed that his men would be going from house to house, room to room, to burn out the opposition," he said.
Armed reconnaissance
"Libyan troops were reportedly committing atrocities in outlying areas of the city. The military intervention... has stopped that attack in its tracks."
At a briefing at the Ministry of Defence, Maj Gen Lorimer set out the latest details of the role British forces had played in the operation.
He said RAF Typhoon jets flying from Gioia del Colle in southern Italy had flown their first mission into "hostile air space", supported by surveillance aircraft and tankers.
He said that, separately, Tornado GR4 ground attack planes had flown armed reconnaissance missions from their base at RAF Marham.
He added they had now gone to the forward base at Gioia del Colle.
No British forces fired weapons in today or last night's operations, the MoD says.
Other details of the operation also emerged from the briefing:
The White House said President Obama has agreed with Prime Minister David Cameron and with French President Nicolas Sarkozy that Nato should play "a key role in the command structure going forward."
Downing Street said they spoke on the telephone on Tuesday evening to review the "substantial progress" in implementing the UN resolution.
But Nato ambassadors have so far not reached agreement on anything except enforcing an arms embargo against Libya.
The debate about whether targeting Col Gaddafi himself would be legal was also continuing in Westminster.
The UN resolution authorised "all necessary measures", short of an occupying force, to protect Libyan citizens from the Gaddafi regime.
No 10 has said it believed it would be legal under certain circumstances to target the Libyan leader, but Britain's Chief of Defence Staff, Gen Sir David Richards, said it was "not allowed under the UN resolution".
'Assassination'
Mr Hague stressed that the coalition should not be choosing the government of Libya.
"That is for the Libyan people themselves," he said. "But they have a far greater chance of making that choice now than they did on Saturday, when the opposition forces were on the verge of defeat."
Former Liberal Democrat leader Sir Menzies Campbell, who is a barrister, said the allies had no legal authority to seek regime change.
He told BBC Radio 4's The World At One: "If Col Gaddafi is, for example, in the lead tank in a column of tanks attacking a town filled with civilians, then that tank and Col Gaddafi would be a legitimate target.
"But he is not a legitimate target from the point of view of assassination."
Libya's government said more civilians were killed in the latest wave of air and missile strikes by coalition forces, but that cannot be independently confirmed.
Fighting is continuing inside Libya between rebels and forces loyal to Col Gaddafi.
There are reports of shelling of the town of Misrata, and of fighting around the central town of Ajdabiya and in Zintan, south-west of Tripoli.
