Israeli air strikes have hit the suburbs of Beirut
Up to 20,000 British and UK-Lebanese citizens in Lebanon have been told to "keep a low profile" amid the crisis in the Middle East.
The Foreign Office has also warned against all travel to Lebanon and urged those already in the area to "get ready for departure at short notice".
Officials have asked the 20,000 living there to register with them or with the British embassy in Beirut.
Tony Blair has called for "calm" in the wake of Israeli air strikes on Lebanon.
The offensive began after the capture of two Israeli soldiers by Hezbollah.
A Foreign Office statement issued on Friday warned: "If you are currently in Lebanon, you should stay put for the time being, exercise caution, keep in touch with the embassy and heed local advice."
Mr Blair called on the entire international community to help resolve the crisis.
He said there had to be energy and commitment from all sides to restart talks on a two-state solution.
Israel has intensified its attacks on Lebanon with fresh strikes on Beirut airport, the road to the Syrian capital and a power plant.
Warplanes also fired at an area where the militant group Hezbollah has its headquarters, and the group's radio station was targeted.
The offensive has killed more than 60 Lebanese civilians since Wednesday.
Hezbollah has continued to fire rockets into Israel, At least two Israelis have been killed by them in the past two days.
Mr Blair, who is flying to Russia for a G8 summit, backed UN moves to "get some restraint into this situation".
Britain was among four nations that abstained when a resolution condemning Israel and demanding it halt its separate military offensive in the Gaza Strip was put to the UN Security Council on Thursday.
The US voted against the resolution.
The Security Council debated the issue at an emergency meeting on Friday and Lebanon called for an end to the Israeli operation.
Mr Blair held a news conference with Canadian PM Stephen Harper
At a news conference with Canadian prime minister Stephen Harper in Downing Street, Mr Blair said he wanted to emphasise "just how serious" he thought the situation in the Middle East was.
"I entirely understand the desire, and indeed need, for Israel to defend itself properly and I also understand the plight of Lebanon and the Lebanese government, not to say the many Palestinians that suffer as well.
"The only way we are going to get this situation resolved is if we support the UN mission, get some calm into the situation and then as soon as possible... get back in to the road map towards a two-state solution that offers the only chance for stability and peace in the future."
Mr Blair said what happened in the past two weeks was an "indication of what will happen every time the energy and commitment of the whole of the international community is not there to try and resolve the situation".
He said the situation could not be allowed to "develop as it is at the moment".
"What is happening is absolutely tragic for all the people involved, but the only solution is that the international community empowers the moderates on both sides to come to a solution."
Mr Blair declined to point the finger of blame at any of those involved in the current crisis.
He said: "We can condemn as much as we like, we are not actually in that situation. We are not having to make the decisions in that situation."
• British and dual nationality citizens contacting the Foreign Office in London should call 020 7008 1500.