At least 45 people have been killed in an air strike that struck a refugee camp in the northern Yemeni region of Haradh according to the International Organization for Migration.

A humanitarian official earlier said the bombing had targeted a military installation in the vicinity of the camp.

A coalition, led by Saudi Arabia, is bombing Houthi militiamen who are advancing on the southern port city of Aden, the last stronghold of President Abd-Rabbu Mansour Hadi.

Pakistan has announced it is to send troops to Saudi Arabia to join the coalition, a senior government official said on Monday.

"We have already pledged full support to Saudi Arabia in its operation against rebels and will join the coalition," the official said.

The Iran-allied militiamen have pushed into the northeastern outskirts of Aden amid heavy clashes with forces loyal to Mr Hadi, sources on both sides said.

Artillery and rocket fire struck the area on the approaches to the city, Government fighters said, after the Houthis made afresh advance from the east along a coast road.

Aden is Mr Hadi's last bastion of control in Yemen and remains besieged despite a fifth day of Saudi-led air strikes aimed at stopping the Houthi gains.

North of Aden, residents in the city of Dhalea said Houthi fighters backed by allied army units loyal to former president Ali Abdullah Saleh shelled militia opponents with tanks and artillery.

Five civilians were killed in heavy street fighting, they said.

Eight civilians have also been killed after rebel forces shelled a town in southern Yemen, according to government officials.

Two children were among the dead and dozens were wounded after troops loyal to Mr Saleh shelled neighbourhoods in Daleh, a town to the north of Aden, using tanks and artillery

Saudi Arabia, backed by regional Sunni Muslim allies, launched an air campaign to support Mr Hadi after he withdrew last month from the capital to Aden.

He left Yemen on Thursday to attend an Arab summit and has not returned.

The fighting has brought civil war to the Arabian Peninsula's poorest country, which was already sliding into chaos with a growing secessionist movement in the south and a covert US drone campaign - now stalled - against Al-Qaeda inthe east.

The growing power of the Houthis, part of a Shia minority that makes up about a third of the population, also means Yemen has become the latest stage for Saudi Arabia's power struggle with Iran.

The two regional rivals support opposing sides in Syria's grinding civil war and in neighbouring Lebanon.

Iran also supports and arms Shia militias in Iraq, although it denies Saudi accusations that it supports Yemen's Houthis militarily.

In the capital Sanaa, controlled by the Houthis, jets struck around the presidential palace next to the diplomatic quarter early today, as well as a weapons depot in the Nugum mountain overlooking the capital.

"It was a night from hell," a Yemeni diplomat said.

Speaking at the Arab summit in Egypt on Saturday, Mr Hadi described the Houthi movement as "Iran's puppet" and accused it of destroying Yemen through what he called its political immaturity.

Mr Hadi left the summit in the Saudi King Salman's plane and is now staying in the Saudi capital.

The Arab leaders agreed at their meeting in Egypt to form a unified military force to counter growing regional security threats such as the Yemen conflict.

But working out the logistics of the force will be a protracted process and Yemen's rugged geography, internal power struggles and recent history all present major challenges to any military campaign.

Just four years after the 1990 unification of North and South Yemen, civil war erupted when southerners tried to breakaway, but were defeated by Mr Saleh's northern forces.

In the 1960s, intervention by Saudi Arabia and Egypt on opposing sides of a civil war in North Yemen led to a long and damaging military stalemate.

Saudi Arabia says it is focusing for now on air strikes against the Houthis, rather than a ground campaign, promising to increase pressure on them over coming days.

It says it has support from eight other Arab states, as well as Pakistan, although it is unclear how much of a role many of those countries are playing.

Pakistan said it had not formally decided what support to offer.