* First reported Qatari ground deployment
* 1,000 troops with vehicles, helicopters - al Jazeera
* Pending Gulf offensive against Houthi forces holding capital
(Updates with report of 12 deaths in fresh air strikes)
SANAA, Sept 7 (Reuters) - Qatar has sent 1,000 ground troops to Yemen, Doha-based Al Jazeera television said, escalating Gulf Arab intervention in Yemen's war ahead of a planned offensive against Iranian-backed Houthis holding the capital Sanaa.
Qatari pilots had already joined months of Saudi-led air strikes on the Houthi militia, which seized Sanaa a year ago and then advanced across much of the country, forcing President Abd-Rabbu Mansour Hadi into exile in March.
Military sources told Reuters that Qatari troops were on their way to Yemen and preparing to join a new push on Houthi positions in Sanaa. They told Reuters the Qatari force had not yet entered the Arabian Peninsula country.
But a regional Yemeni official in oil-producing Marib province east of Sanaa said the Qatari contingent had already "crossed the al-Wadia border post" between Saudi Arabia and Yemen and was heading to Marib - where Hadi loyalists have been preparing for the thrust towards Sanaa. .
Saudi-owned al Arabiya satellite network also said Qatari and Saudi reinforcements had crossed the frontier.
The first reported involvement of Qatari ground forces in Yemen coincided with an intensification of the conflict a few days after a rocket strike in Marib that killed dozens of soldiers including Saudis and Emiratis.
Saudi coalition forces on Sunday carried out repeated air raids on Houthi targets and allied troops loyal to former president Ali Abdullah Saleh in apparent retaliation.
Saudi-led warplanes on Monday renewed strikes on Houthi targets across Yemen and Houthi-run media said the raids killed at least 12 people, including women and children, in the central province of Ibb.
It was not immediately possible to independently verify the report.
ARMOURED VEHICLES, APACHE HELICOPTERS
Al Jazeera's English website said 1,000 Qatari soldiers, backed by 200 armoured vehicles and 30 U.S.-made Apache helicopters had been deployed.
Qatar's foreign ministry made no immediate comment.
A Qatar-based defence source said the number of Qatari troops was less than 1,000. "They are as of now not deployed in Yemen but in Saudi Arabia to protect the border," he added.
The Saudi-owned al-Hayat newspaper said on Monday that Saudi Arabia had also sent "huge reinforcements" of elite forces, along with Qatari troops, to Marib.
"Final preparations are being made for a decisive battle, before moving on to liberate Sanaa," al-Hayat said.
Jean-Marc Rickli, a professor at King's College London who is teaching at Qatar National Defence College, told Reuters it was Qatar's first deployment of ground forces in Yemen.
"This force will probably take part in the overall war effort to retake the capital after the coalition successfully recaptured Aden last month," he added.
Gulf Arab states regard the Houthis as proxies for non-Arab Iran, which they accuse of trying to extend its influence into Arab countries, including Syria and Yemen.
Saudi-led forces have helped Hadi supporters drive the Houthis out of the southern port of Aden in July but have made little progress in other areas since, where the fighting in Marib and the central city of Taiz remains bogged down.
(Reporting by Yara Bayoumy in Dubai, Tom Finn in Doha, and Mohammed Ghobari in Sanaa; Writing by Sami Aboudi; Editing by Andrew Heavens) ((Sami.Aboudi@thomsonreuters.com; +97143918301;))