"This is a different Middle East. It's a new Middle East. It's hard, We're going through a very violent time," the US secretary of state said.
"A ceasefire would be a false promise if it simply returns us to the status quo.
"Such a step would allow terrorists to launch attacks at the time and terms of their choosing and to threaten innocent people, Arab and Israeli, throughout the region."
She was speaking on Saturday after meeting with members of a United Nations team that had just returned from the region.
More than 300 Lebanese civilians have been killed in 11 days of Israeli air and artillery strikes against Hezbollah, the armed Lebanese Shia group.
The present round of fighting between Israel and Hezbollah broke out after Hezbollah kidnapped two Israeli soldiers in a cross-border raid on July 12.
Rice will travel to the Middle East some time next week. Her itinerary has not yet been announced.
Call for negotiations
Hezbollah has offered to release the two soldiers if Israel frees Palestinian prisoners held by Israel.
But Israel has said that Hezbollah must release the soldiers unconditionally and says that it will not halt its offensive until Lebanon implements UN resolution 1559 and disarms Hezbollah.
Since then the Israeli aircraft have carried out more than 3,000 sorties in Lebanon and have bombed infrastructure including bridges, roads and ports.
Rice met the Lebanese prime minister in February
By Saturday morning the attacks had killed at least 305 Lebanese civilians and at least 12 members of the Lebanese army, which has tried to stay out of the conflict.
Israeli military leaders say that the attacks are necessary to stop Hezbollah firing rockets at Israel.
Since fighting started 11 days ago, Hezbollah has fired more than 1,000 rockets at Israel, killing 15 civilians.
"Many rocket launchers have been destroyed, terror infrastructures have been destroyed and also nearly 100 Hezbollah terrorists have been killed, from all levels and all ranks," Dan Halutz, the Israeli army's chief of staff, said on Friday.
"Fighting against Hezbollah is taking a heavy toll on [the group].
"The fact is that they avoid publishing the number of their losses, the names of their men that were killed, and the fact that they feed the press dishonest information [shows] they are disconnected from reality."
The Israeli military has also announced that for the last three days its troops have held positions inside Lebanon near the villages of Marwaheen in the west and in Maroun al-Ras further east.
"At most, we're talking about a few kilometers in," an Israeli army spokesman said.
So far Israel has mainly attacked Hezbollah from the air
"It will probably widen, but we are still looking at limited operations. We're not talking about massive forces going inside at this point."
On Friday, the Israeli army called up 3,000 reserve soldiers and began massing troops and armoured vehicles near the Lebanese border.
An Israeli spokesman, Captain Yaacov Dalal, said ground operations were "indispensable because the air force can not always destroy underground bunkers dug by Hezbollah, which has put in place an entire fortified network".
New air strikes
The Israeli army also said it attacked more than 150 targets across Lebanon in the last 24 hours, including a Hezbollah weapons bunker, command posts and 11 rocket launchers.
An Aljazeera television correspondent in Lebanon reported that overnight Israeli airstrikes had hit residential areas in the southern town of Nabatiya, killing one person and wounding eight others.
Other air strikes had hit the town centre of Al-Khaim, damaging many houses on Saturday morning, she said.
In northern Israel two people were wounded when 10 Hezbollah rockets landed in and around the town of Carmiel. Other rockets hit the towns of Nahariya and Rosh Pina, closer to the Lebanese border, medics said.