Mr Clinton called for an immediate ceasefire. "Today's events make painfully clear the importance of bringing to an end the current violence in Lebanon," he said as he arrived in Russia after a flight from Tokyo.
As the US President spoke, Ehud Barak, the Israeli foreign minister, said of the massacre at Qana: "I am sure that along with everyone else we are very sorry about any harm done to civilians."
Later, Israeli Prime Minister Shimon Peres said he would heed President Clinton's ceasefire call if Hizbollah guerrillas also agreed. "Israel will accept the call of the President of the United States. If the other parties will agree to a ceasefire, we shall agree immediately," he told CNN.
As a first step to securing peace, Washington has already dispatched Dennis Ross, the State Department's top Middle East specialist, to the region. Mr Christopher is expected to follow him within 48 hours to put what the US hopes will be the finishing touches to a ceasefire arrangement acceptable to both Israel and Lebanon (and by implication Syria, de facto protector of the Hizbollah).
The rescheduling means that Mr Christopher, 70 - who constructed a similar truce between Israel and Hizbollah in 1993 - will travel to the Middle East directly from Europe, where he is due to meet the Chinese Foreign Minister, Qian Qichen at the Hague today, before attending a US-Russian summit.
The Israeli attack could not have come at a worse time for Washington. Not only does it threaten to change the entire dynamic of the faltering overall peace process; it took place as President Clinton was en route from Japan to Moscow, where the incident could dominate a visit that had been orchestrated to boost President Yeltsin's chances of re-election.
Now however Washington is contemplating the grim possibility that its Middle East peacemaking efforts may unravel. Despite a UN statement that 15 minutes before the camp was shelled Hizbollah had launched Katyusha rockets and mortars from a site just 300 yards away, the slaughter is shaping up as an international public relations disaster for Israel.
It demonstrates moreover the lack of US control over its protege; days before yesterday's developments, Washington had been seeking to end the fighting and even the New York Times, a staunch supporter of Israel, argued that Jerusalem should halt its offensive before it plays irretrievably into Hizbollah's hands.
That, analysts here say, is precisely what will happen now, re-inforcing the militant Arab argument that Israel is waging not a limited war against guerrillas but a general war against ordinary civilians, and streng thening calls for it to withdraw from its "security zone" in southern Lebanon.
Sympathy for Hizbollah will only grow, intensifying international pressure on Israel to make concessions in any ceasefire deal. That however is less likely than ever the run-up to next month's Israeli elections, in which the Labour Prime Minister Shimon Peres is out to show he is as tough on terrorism as anyone.
Terms for a truce, for which earlier separate American and French initiatives are now jointly working, will almost certainly have to involve some future commitment by Israel to pull out of southern Lebanon, in return for an end to bombardment by Hizbollah units of northern Israel.
But as US diplomats know full well, the involvement of Syria, which has 35,000 troops in Lebanon and thus allows weapons and supplies to reach the guerrillas, is crucial. Indirectly therefore, a resolution of the current crisis could restart Israeli-Syrian peace talks.