A popular Yemeni writer with a large Twitter following has been held incommunicado by Saudi authorities for months, human rights groups said Wednesday, calling for his immediate release.

Marwan al-Muraisy, a social media activist with more than 100,000 Twitter followers, was arrested at his home in Riyadh on June 1, Reporters Without Borders (RSF) and other watchdogs said in a joint statement.

War crimes investigators called Wednesday for the UN's top rights body to let them continue probing the "extremely alarming" situation in Yemen, amid resistance from Saudi Arabia and others.

The Sunni-ruled Gulf state of Bahrain has arrested 15 people it accuses of attempting to "cause chaos" during Shiite religious commemorations that culminated last Thursday, police said.

The United Arab Emirates, whose troops are fighting Yemeni rebels alongside government forces, said Wednesday it would fully support new peace talks after UN-brokered negotiations broke down earlier this month.

UAE state minister for foreign affairs Anwar Gargash said he had had "very productive discussions" with UN Yemen envoy Martin Griffiths on the sidelines of the UN General Assembly in New York.

Six months after President Donald Trump said he wants US troops out of Syria, his top officials are hammering home what has become increasingly obvious: the US isn't going anywhere.

Trump administration members say there can be no troop pull-out until the Islamic State is permanently defeated -- a subjective metric for a stubborn insurgency where the jihadists have shown tenacity in clinging to their last pockets of terrain.

President Donald Trump warned Tuesday that the U.S. would respond to any new chemical weapons attack by Syrian leader Bashar al-Assad's regime as he called for fresh momentum to be injected into international peace efforts

"The ongoing tragedy in Syria is heartbreaking. Our shared goals must be the de-escalation of military conflict, along with a political solution that honors the will of the Syrian people," Trump said in a speech at the United Nations General Assembly.

Israeli Prime Minister Benjamin Netanyahu pledged Tuesday to continue combating Iranian "entrenchment" in Syria and coordinating with Russia after the downing of one of Moscow's planes.

A senior Hamas official denied Tuesday that Egyptian-brokered talks on reconciliation with the Palestinian Authority and a lasting truce with Israel have collapsed, but admitted progress was slow.

"The efforts of our Egyptian brothers continue on the file of Palestinian reconciliation and the calm with the (Israeli) occupation," spokesman Sami Abu Zuhri told AFP.

Turkish troop reinforcements entered Syria's rebel bastion of Idlib on Tuesday, an AFP correspondent reported, a week after a deal between Ankara and Moscow averted a government offensive.

Around 35 military vehicles travelled south down the main highway near the town of Saraqib after midnight.

Human Rights Watch on Tuesday accused Yemen's Huthi rebels of hostage-taking, torture and other serious abuses against people in their custody.