RIYADH (AFP) - Yemeni rebel "suicide" boats attacked a Saudi warship on patrol in the Red Sea, killing two sailors in a rare naval clash in the nearly two-year-old war, the Saudi-led coalition said.

The assault off the rebel-held port city of Hodeida came as government forces backed by the coalition pressed a deadly drive up the Red Sea coast despite mounting international pressure for a ceasefire.

"A Saudi frigate came under a terrorist attack by three suicide boats belonging to the Huthi militias," the coalition said late on Monday (Jan 30) without specifying when the incident occurred.

Suicide attacks are uncharacteristic of the Shi'ite Muslim Huthi rebels. They are normally the work of Sunni extremists of Al-Qaeda or the Islamic State in Iraq and Syria (ISIS).

Although the Saudi warship "dealt with the boats", one of them "collided with the back of the ship and exploded and caused a fire", which the crew brought under control, the coalition said.

As a result of the incident, two Saudi crewmen lost their lives and three were wounded, the coalition said, adding the frigate was able to resume its patrol.

In purported video of the attack shown on the rebels' Al-Masirah television website, the stern of a warship exploded in a large fireball.

The rebels claimed responsibility for the attack without specifying how the vessel was targeted.

"It was hit with precision after an accurate surveillance operation off the western coast," a rebel military official said in a statement.

In October the rebels fired rockets towards an American destroyer, prompting the US Navy to retaliate with cruise missiles against mobile radar sites in Huthi-controlled territory.

Since it began air strikes in March 2015, the coalition has imposed an air and sea blockade of rebel-held areas.

It has carried out patrols of the Red Sea to prevent what it says is attempted arms smuggling to the rebels by Shi'ite Iran.

Before government forces launched a major offensive on Jan 7, the rebels controlled virtually all of Yemen's 450km long Red Sea coastline.

But loyalist forces have since thrust north from the Bab al-Mandab strait where the Red Sea joins the Indian Ocean, overrunning Dhubab district and entering the historic port of Mokha in their biggest advance in months.

There have been heavy losses on both sides. Nearly 370 combatants have been killed, according to medical sources.

Government forces have also pushed south from a small pocket of territory they control around the port of Midi near the Saudi border.

Heavy fighting has raged around both Midi and the inland town of Haradh, leaving 21 government troops and seven rebels dead over the past 24 hours, military sources said on Tuesday (Jan 31).

Loyalist forces launched an assault on rebel positions but found themselves in a minefield where they came under heavy gunfire, a military official said.

"That is why the death toll was high."

The offensive comes with President Abedrabbo Mansour Hadi under mounting international pressure to agree to a UN ceasefire plan.

In a speech to the Security Council on Thursday (Jan 26), UN envoy Ismail Ould Cheikh Ahmed criticised Mr Hadi for rejecting his proposals for a transition that would see him cede much of his power to a vice president who would oversee a government of national unity.

"President Hadi continues to criticise the proposals without agreeing to discuss them and this will hinder and impede the path towards peace," the envoy said.

Foreign diplomats have for months privately articulated similar concerns about Mr Hadi, who is backed by the coalition.

On Monday (Jan 30) rebel rocket fire against "a United Nations building" in South Dhahran city wounded a Saudi soldier, the official Saudi Press Agency said.

The building is supposed to host meetings of a commission facilitating contact between Yemen's warring sides.