RAMALLAH: Rival Palestinian factions Fatah and Hamas pledged unity against Israel's West Bank annexation plans in a rare joint conference Thursday, as signs emerged of a rift between Israel and Washington over the project.

The relationship between Fatah, which controls the Ramallah-based Palestinian Authority, and Islamist group Hamas - in control of the Gaza Strip - has been plagued by divisions for more than a decade.

The joint news conference was spurred by common opposition against US President Donald Trump's controversial peace plan, which paves the way for Israel to annex territory in the occupied West Bank, including Jewish settlements considered illegal under international law.

"We will put in place all necessary measures to ensure national unity" in efforts against annexation, senior Fatah official Jibril Rajub said in Ramallah at the news conference, also addressed by Hamas official Saleh al-Arouri by video-link from Beirut.

Arouri described the conference as "an opportunity to start a new phase". Hamas chief Ismail Haniyeh said in a statement that the event marked "a major step towards unity", which was endorsed by "the leadership of Hamas".

But for former PA official and Palestinian analyst Ghassan Khatib, Thursday's show of unity was unlikely to spark wider co-operation between the two Palestinian groups. "I doubt the annexation challenge will help these two factions to end their split and unify again.

I don't think this is going to happen," Khatib told AFP. "They'll agree about the significance and the importance of the annexation and the need to try to coordinate their efforts but I don't think they'll go beyond that," he added.

Prime Minister Benjamin Netanyahu's centre-right coalition government had set July 1 as the date it could begin implementing Trump's annexation proposals. But the US plan, unveiled in January, also calls for annexations to come as part of a larger peace package, including negotiations on the creation of a demilitarised Palestinian state across roughly 70 percent of the West Bank, with a link to Gaza.