BEIRUT, July 24 (BBC): Condolizza Rice is expected to meet Lebanese leaders, including PM Fouad Siniora.
En route from Washington, Ms Rice said there was an "urgent" need for a ceasefire in Lebanon - but that conditions had to be right.
Ms Rice said there must be no place for "terrorist groups" like Hezbollah to launch attacks from Lebanese territory.
Ms Rice will later head to Israel to meet Prime Minister Ehud Olmert.
Heavy fighting is reported in southern Lebanon, amid further Israeli air strikes and Hezbollah rocket attacks.
At least 362 Lebanese, the great majority civilians, have been killed during the conflict, which is now into its 13th day. Thirty-seven Israelis have been killed, about half of them civilians.
The Israeli offensive began after Hezbollah captured two Israeli soldiers in a cross-border raid on 12 July.
During her plane journey from Washington, Ms Rice told reporters Hezbollah must no longer be allowed to "plunge Lebanon and the region into war".
"It is very important to establish conditions under which a ceasefire can take place," she said.
It is important to have conditions that will make it also sustainable."
But Ms Rice's admission that an end to the fighting is "urgently" wanted marks a shift from previous statements in which she said an immediate ceasefire would only offer "false promise", say correspondents.
In a policy change, Israel's Prime Minister Ehud Olmert has said Israel would be prepared to accept a European peacekeeping force in southern Lebanon providing it is robust and has a strong mandate The Israelis are hinting at a more realistic assessment of what they can achieve through the application of brute force alone, BBC diplomatic correspondent Paul Adams reports from Jerusalem.
The BBC's Kim Ghattas, in Beirut, says Lebanon has felt let down by the international community because there has been no call for an unconditional ceasefire.
However, Lebanese Prime Minister Fouad Siniora has also said there would be no return to the status quo before the violence started. And he insisted the Lebanese government was trying to extend its authority over the whole of the country
Meanwhile, fighting between Israeli soldiers and Lebanese fighters continues on the ground.
Having captured the village of Maroun al-Ras in the very south of Lebanon, Israeli forces appear to be pushing northwards, reports the BBC's Bethany Bell, and fierce clashes are reported near the town of Bincha Bail.
Despite the diplomatic moves, Israeli Brig Gen Alon Friedman told Israel Army Radio the ground operation would likely go on for another 10 days.
Correspondents say the US is unlikely to push for an immediate end to the Israeli assault, and one aim of Ms Rice's trip is to assess how much time is needed for Israel to make a significant impact on Hezbollah capability
Israel, which pulled out of southern Lebanon in 2000, has vowed to destroy Hezbollah's ability to launch rockets at its territory.
But Hezbollah leader Sheikh Hassan Nasrallah said an Israeli invasion of southern Lebanon would not succeed.
"Any Israeli incursion will not have political results unless it achieves any of the announced goals, most importantly to stop the bombardment of Zionist settlements," he told As-Safir newspaper.
"I assure you that this will goal will not be achieved."