The UK "must prepare for the long haul" in Libya, Downing Street has said.
But Foreign Secretary William Hague said the conflict had "not settled into a stalemate", and "time was not on the side" of Muammar Gaddafi.
He added that the UK had so far given £13m of aid to Libya, including food for 10,000 people in the besieged city of Misrata.
Defence Secretary Liam Fox cited new momentum in the campaign, and said Col Gaddafi was "on the back foot".
The Ministry of Defence reported that RAF fighter aircraft had "successfully attacked" three armoured personnel carriers near Misrata over the weekend.
And a Nato air strike has badly damaged buildings in Col Gaddafi's compound in Tripoli.
Mr Hague has briefed the cabinet about the situation in Libya and Dr Fox held talks on Libya at the Pentagon with his US counterpart Robert Gates.
Also, UK Chief of the Defence Staff General Sir David Richards met his counterpart Adm Mike Mullen, chairman of the joint chiefs of staff.
Asked about a Nato air strike on Monday on Col Gaddafi's compound in Libya, Mr Gates and Dr Fox told reporters command and control centres were "legitimate" targets, though Mr Gates said Nato was not targeting Col Gaddafi specifically.
Last week, Adm Mullen said the war in Libya was "moving towards stalemate" despite the destruction of 30-40% of Col Gaddafi's ground forces.
Asked about those comments by shadow foreign secretary Douglas Alexander, Mr Hague told the House of Commons that, while the situation in eastern Libya was somewhat "static", the overall situation was not a stalemate.
He said: "We are often asked in international conflicts whether time is on our side.
"We should be confident that in this situation, given this coalition, this range of sanctions, these intensifying efforts, time is not on the side of Gaddafi and the members of his regime need to know that."
Mr Hague was asked whether he would agree to another Commons debate and vote on Libyan intervention, but he refused.
"I don't think the government's policy on the matter has changed in any material way that requires a fresh vote," he said.
'No step change'
Mr Alexander said he supported moves announced by the government during Parliament's recess to assist the rebel fighters, including the provision of body armour and communications equipment.
But he added: "The ad hoc and apparently uncoordinated manner in which they were announced, rooted in no clearly articulated plan, has, I fear, only served to increase anxieties held by many members of the public."
Earlier, a spokeswoman for the prime minister said the UK must be prepared for a long-term commitment in Libya, but insisted that was not "a step change" from previous government statements.
"The term 'for the long haul' underlines our commitment to the UN resolution and the people of Libya," she said, and was "a common sense interpretation of where we are".