Saudi-backed loyalist forces in Yemen pressed an offensive against Shiite Huthi rebels on Monday, the second day of a major campaign aimed at retaking the capital a year after its fall.

Fighters loyal to exiled President Abedrabbo Mansour Hadi launched their much anticipated attack against the Iranian-backed rebels and their allies in the key province of Marib.

Saudi-led coalition warplanes heavily bombarded rebel positions across the Yemeni capital Saturday, ahead of expected U.N.-mediated peace talks, witnesses said.

They targeted arms depots and military camps in the rebel-held capital's northern districts.

Hundreds of rebels protested Friday in Sanaa over claims that Saudi Arabia, home to Islam's holiest sites and host to the hajj, was denying Yemenis the right to attend this year's pilgrimage.

Dressed in the white shrouds worn by pilgrims and brandishing assault rifles, they shouted "No to politicizing the hajj."

Iran-backed rebels fired Katyusha rockets at a busy market in Yemen's eastern city of Marib killing at least 20 people on Friday, medics and witnesses said.

"Twenty civilians were killed and dozens wounded" in the attack, a medical official said. Witnesses said the provincial capital's market was targeted.

Warplanes of the Saudi-led coalition on Wednesday bombed Shiite Huthi rebel positions in Sanaa, stepping up attacks in the wake of a deadly missile strike on its troops, witnesses said.

They said the latest raids targeted arms depots held by the Huthi rebels and renegade troops loyal to ousted president Ali Abdullah Saleh in southeast Sanaa. 

Security forces have arrested a suspect in connection with the shooting death of two Red Cross workers in a rebel-controlled area of Yemen, Saba news agency reported Thursday.

The suspect was arrested on Wednesday night in Amran province north of Sanaa, hours after an assailant gunned down the two Yemenis working for the International Committee of the Red Cross, Saba said.

The Islamic State group claimed twin bombings of a Shiite mosque that killed at least 28 people and wounded 75 in the Yemeni capital Wednesday.

The Sunni extremist group, which has claimed similar bombings in the past, said a man named Qusai al-Sanaani blew himself inside the mosque and that a bomb-laden vehicle parked nearby subsequently exploded.

A gunman killed two Yemenis working for the International Committee of the Red Cross in the war-torn country Wednesday, the aid group said, condemning the attack as "appalling" and "brutal."

The attack appeared to be deliberate as the pair had been traveling north of the capital Sanaa with two other colleagues in vehicles "clearly" marked with the Red Cross emblem, said the Geneva-based ICRC.

Seven pro-government activists were heading to Yemen's main southern city of Aden on Monday as part of a prisoner swap overseen by the International Committee of the Red Cross, rebel officials said.

The seven activists belong to the Southern Movement, whose militants were locked in deadly battles with Shiite Huthi rebels, who they helped to push out of Aden in mid-July.

Pro-government forces in Yemen strengthened by tanks newly supplied by a Saudi-led coalition launched an offensive Saturday to retake the rebel-held capital of Abyan province, military sources said.

A two-pronged drive on the southern province's capital of Zinjibar was launched from the north and south, the sources said.