Syria and Iran should stop their support for Hezbollah and end their interference in Lebanese affairs, the UK foreign secretary has said.
Mrs Beckett says the UK is 'gravely concerned' by the crisis
Margaret Beckett said the two states' support for Hezbollah was "encouraging extremism" and putting peace in the Middle East "further out of reach".
Even if the crisis was not engineered by Syria and Iran, she said, its timing was "remarkably convenient" for them.
She said every effort was being made to achieve a "durable" ceasefire.
Mrs Beckett was speaking in the Commons as US Secretary of State Condoleezza Rice held talks in Jerusalem with Israeli PM Ehud Olmert in an attempt to ease Israel's conflict with Lebanon.
But Mrs Beckett resisted calls from MPs to take a firmer line as Israeli forces kept up their offensive in Lebanon.
She acknowledged events there were "a tragedy" for ordinary Lebanese people but said Hezbollah was "deliberately siting missiles in the heart of civilian populations".
She added: "It is bound to cause difficulty when those missiles are continually raining down on Israel and clearly there is a pressure on Israel to attempt to take out those missile sites.
"This is a very strong contributory factor to the terrible events which are indeed taking place in Lebanon."
She rejected Tory MP Sir Peter Tapsell's accusation that Britain had been in "collusion" with the US to allow Israel to commit war crimes.
And she stressed that it could take some time to deploy an international force in southern Lebanon.
Shadow foreign secretary William Hague said Israel's demands - while "wholly legitimate" - would be greatly strengthened by stopping attacks on civil infrastructure in Lebanon.
He added: "On the international buffer force that has been proposed, given that British troops are not available and American troops perhaps not appropriate and the French have said the idea is premature, are you confident there can be put in place a sufficiently capable and well-equipped force when necessary?"
Mrs Beckett replied: "The use of the words buffer force is not something that is falling happily on people's ears in the area."
But she added: "You are certainly right to identify both that this will be time consuming and not easy and the whole issue of the mandate of such a force and the nature and construction of such a force is going to be a matter of difficult negotiation.
"I expect that will be very much a part of the discussions in Rome tomorrow (Wednesday).
"But that is why I believe that one of the strong efforts that we must make is to see what can be done now to ameliorate the situation."
In the short term this would probably be primarily through humanitarian means, she added.
Liberal Democrat foreign affairs spokesman Michael Moore welcomed the "hard hitting comments" of Foreign Office Minister Kim Howells during his visit to the region at the weekend.
Mr Howells, who also criticised Hezbollah, urged Israel to show "proportionality and restraint" and to "think very hard about those children who are dying".
Mr Blair and Mrs Beckett have so far refused to criticise the impact of the Israeli bombing on Lebanon, said Mr Moore.
"We would go further and repeat the view, as Kofi Annan has said, that there ought to be an immediate ceasefire," he added.
And he warned: "For as long as the United States, and by extension the United Kingdom, tolerate the disproportionate response by the Israelis the diplomatic efforts will be undermined completely."