(Reuters) - Yemen's Houthi rebels made broad gains in the country's south and east on Friday despite a second day of Saudi-led air strikes meant to check the Iranian-backed militia's efforts to overthrow President Abd-Rabbu Mansour Hadi.

Shi'ite Muslim Houthi fighters and allied army units gained their first foothold on Yemen's Arabian Sea coast by seizing the port of Shaqra 100km (60 miles) east of Aden, residents told Reuters.

Explosions and crackles of small gunfire rang out across Aden late on Friday as Houthis made a push on the southern port city's airport, a witness said.

The advances threaten Hadi's last refuge in Yemen and potentially undermine the air campaign to support him.

The spokesman for the Saudi-led operation, Brigadier General Ahmed Asseri, told a news conference in Riyadh that defending the Aden government was the campaign's "main objective".

"The operation will continue as long as there is a need for it to continue," Asseri said.

Warplanes targeted Houthi forces controlling Yemen's capital Sanaa and their northern heartland on Friday. Asseri said that planes from the United Arab Emirates had carried out their first strikes in the past 24 hours.

In a boost for Saudi Arabia, Morocco said it would join the rapidly assembled Sunni Muslim coalition against the Houthis. Pakistan, named by Saudi Arabia as a partner, said it had made no decision on whether to contribute.

REGIONAL CONTEST

Riyadh’s military intervention is the latest front in a growing regional contest for power withIran that is also playing out in Syria, where Tehran backs Assad’s government against mainly Sunni rebels, and Iraq, where Iranian-backed Shi’ite militias are playing a major role in fighting.

Sunni monarchies in the Gulf are backing Hadi and his fellow Sunnis in the country's south against the Shi'ite advance.

Yemen's powerful ex-president Ali Abdullah Saleh, whose military units fight alongside the Houthis, called on Friday for a cessation of hostilities by both sides, according to a statement carried by his party's website.

Yemeni Foreign Minister Riyadh Yaseen said the air campaign could end within days.

He said the door was still open for dialogue with the Houthis, while in a Facebook posting, Hadi urged Yemenis to be patient and predicted the Houthis would soon be gone.

But the Houthis and allied army units seized the southern town of Shaqra in Abyan province on Friday, gaining access to the Arabian Sea, residents said.

Their entry into the city means they control most land routes to Aden and can block tribal fighters trying to come in to reinforce Hadi's troops.

Residents said dozens of pickup trucks loaded with tribal fighters have reached the town of Mudyah and were expected to clash with the Houthi forces based in Shaqra and the town of Lodar.

During a week of intense fighting, the Houthis have taken the Red Sea port of al-Mukha to Aden's northwest, and the city's northern outskirts, suggesting Aden is danger, despite the air strikes against the Houthis.

Witnesses in Sanaa said Houthi fighters and allied military units were re-positioning some anti-aircraft units at police stations in some neighborhoods, causing panic among residents, who fear they will become targets for air strikes. Residents said aircraft targeted bases around Sanaa of Republican Guards allied to the Houthis, and also struck near a military installation that houses missiles. The Houthi-controlled Saba news agency put the death toll in Sanaa at 24 and said 43 were wounded and 14 houses were destroyed.

Houthi-run al-Masirah television also said 15 people were killed in an air strike on a market in the northern city of Saada.

OIL REGION HIT

The Republican Guards are loyal to Saleh, who retains wide power despite having left office in 2012 after mass protests.

Earlier air strikes south of the city and in the oil-producing Marib region appeared to target military installations also affiliated with Saleh.

Warplanes also hit two districts in the Houthis' northerly home province of Saada, tribal sources said.

The coalition began air strikes on Thursday to try to roll back Houthi gains and shore up Hadi, who has been holed up in Aden after fleeing Sanaa in February.

Hadi left Aden on Thursday to attend an Arab summit in Egypt on Saturday, where he aims to build support for the air strikes.

U.S. President Barack Obama expressed his support for the Saudi-led military action in a phone call with Saudi King Salman on Friday, the White House said.

In his first reaction to the attacks, Houthi leader Abdel-Malek al-Houthi on Thursday calledSaudi Arabia a bad neighbor and "Satan's horn", saying in a televised speech Yemenis would confront the "criminal, unjust and unjustified aggression".

Mosques in Riyadh on Friday preached fiery sermons against the Houthis and their Iranian allies, describing the fight as a religious duty. Saudi Arabia's top clerical council gave its blessing to the campaign.

In the Iranian capital Tehran, Friday prayer leader Ayatollah Kazem Sadeghi described the attacks as "an aggression and interference in Yemen’s internal affairs".

Iran has denounced the assault on the Houthis and demanded an immediate halt to Saudi-led military operations.

While U.S. officials have downplayed the scope of the ties between Iran and the Houthis, Saudi ambassador to Washington Adel al-Jubeir said members of Iran’s Revolutionary Guards and Iranian-backed Hezbollah are on the ground advising the Houthis.

The Saudi military spokesman said there were no plans at this stage for ground force operations, but if the need arose, Saudi and allied ground forces would repel "any aggression."

By Reuters