Hamas and Islamic Jihad have said they are not bound by the ceasefire agreed between Palestinian and Israeli leaders at a summit in Egypt on Tuesday.
The two militant groups were at the forefront of attacks on Israel during the Palestinian uprising, but have been observing an unofficial truce.
The ceasefire is designed to end four years of Israeli-Palestinian violence.
Israel is expected to free hundreds of Palestinian prisoners shortly, and hand over control of some West Bank towns.
Palestinian leader Mahmoud Abbas said on Wednesday that Israel had agreed to remove major roadblocks as part of its withdrawal from the five towns in the coming weeks.
Meanwhile, a high-level meeting of the “quartet” tasked with advancing the Middle East peace process – the US, the European Union, Russia, and the United Nations – will be held in London on 1 March.
The gathering will occur during a conference promoting steps to Palestinian statehood, set up by UK Prime Minister Tony Blair.
Hamas representative Osama Hamdan said in Lebanon that the ceasefire declarations were “not binding” on Hamas members.
“We agreed with [Mahmoud Abbas] that any truce that should take place must be according to the result of an inter-Palestinian dialogue,” he told the Associated Press news agency.
“The resistance is not committed to what has been agreed at the summit… since the Palestinian conditions were not achieved.
“We consider that there is no truce and there is no deal to stop the resistance.”
An Islamic Jihad spokesman, Mohammed al-Hindi, said the summit had brought “nothing new”.
However, Palestinian Foreign Minister Nabil Shaath said he was “fully content” that compliance with the ceasefire would be “complete and comprehensive”.
The BBC’s Alan Johnston, in the Gaza Strip, says Mr Abbas has effectively said to the militants that he can achieve more through dialogue and negotiation than they could through their bombings and missile attacks.
He adds that if Mr Abbas cannot deliver, the likes of Hamas will argue that Israel only listens to the language of the rocket and the Kalashnikov.