Yemen's dominant Houthi group and its army allies fired a Scud missile at Saudi Arabia which the kingdom says it shot down on Saturday, in a major escalation of two months of war.

In the first use of the long range ballistic Scud in the conflict, the missile was fired early Saturday morning at the city of Khamees Mushait in the kingdom's southwest and was intercepted by two Patriot missiles, a statement by the Saudi military said.

The area is home to the largest air force base in southern Saudi Arabia, the world's top oil exporter, but there are no oil facilities in the vicinity.

Al Masira, the Houthi group's official channel, confirmed the launch and said it targeted the Prince Khaled air base.

An alliance of Gulf Arab nations has been bombing Yemen's Houthi militia and allied army units loyal to powerful ex-President Ali Abdullah Saleh since March 26 in an attempt to restore exiled President Abd-Rabbu Mansour Hadi to power.

The coalition has said a main goal of its war effort is to neutralise the threat that rockets in Yemen pose to Saudi Arabia and its neighbours.

A Saudi spokesman for the coalition said in April that the alliance had succeeded in removing the threat of heavy weapons to the Kingdom and its neighbours, but the air war and border clashes have persisted.

The Sunni Muslim coalition states also fear the Houthis, hailing from a Shi'ite sect in Yemen's far north, will act as a proxy for the influence of their arch-rival, Shi'ite Iran, in the Arabian Peninsula.

Iran and the Houthis deny any military or economic links, and the Houthis say their seizure of the capital Sanaa in September and their advance southward is part of a revolution against a corrupt government.

Arab air strikes have pounded arms and missile stores in the capital Sanaa and other military bases in Yemen almost every day, but the firing of the Scud -- an 11-metre (35-foot) long ballistic missile with ranges of 300 km (200 miles) and more -- shows the country's supply has not yet been eliminated.

Saleh, Yemen's autocrat president from 1978 to 2012, was forced to step down amid Arab Spring street protests but retains most of the army's loyalty and has joined forces with the Houthis in combat with Hadi's armed backers in Yemen's south.

Saudi-owned Al Arabiya TV described overnight ground fighting along the border as the "largest attack" yet by Houthi forces and Yemen's republican guard, a unit close to Saleh.

"It was the first confrontation undertaken by Saleh's (Republican) guard, and coalition planes and Saudi Apache (helicopters) undertook ground fire for 10 hours," said Al Arabiya's correspondent in the southern Jizan region.

Hamed al-Bukhaiti, a Houthi spokesman, indicated that the group had embarked on an escalation along the border.

"We've only just begun, and next time will be stronger," he said on his twitter page.

The options are open and the battle has begun to block the aggression on Yemen ... this is the battle the people of Yemen have been awaiting," he wrote.

Saudi-led forces said on Friday that four Saudi troops, including an officer, were killed after an attack was launched from the Yemeni side on border areas in Jizan and Najran.

Residents in the southern city of Aden said heavy artillery battles resumed after a pause of several days on Saturday, in clashes which killed around 10 Houthi fighters and three pro-Hadi militiamen.

Eyewitnesses said around 10 Arab air strikes pounded Houthi positions in Aden's northwest suburbs on Saturday morning.

The violence comes despite progress toward United Nations-backed peace talks planned for Geneva this month, to which both the exiled government and the Houthis have agreed.-Reuters