Arab special forces have reportedly entered the southern port city of Aden.
(Transcript from World News Radio)
Witnesses in Yemen say between 40 and 50 Arab special forces have entered the southern port city of Aden.
A local newspaper has reported they have already engaged in fighting Houthi forces.
Yemenis say the fighters are supporting forces of exiled president Abd Rabbuh Mansur Hadi.
But Saudi Arabia denies it has sent in troops.
Gary Cox reports.
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New video aired on the Al Jazeera network showing fighters on the streets of Aden has raised eyebrows.
The fighters' clothing is consistent with those worn by combatants in the area.
But a lecturer in defence studies at Kings College in London, Jean Marc Rickli, says they're carrying unusually sophisticated weapons.
"It is probably an anti-tank weapon used by light infantry troops. it's a typical weapon used by special operation forces. But at the end of the day you have to define your objective. and then find the best way to achieve it so going with a ground operation would not be the wisest way to do it because it would be very costly."
The war is intensifying in Aden, and some local fighters believe they are Saudi special forces on a reconnaissance mission.
But the Saudi coalition spokesman Ahmed Asseri denies it.
"I can assure you that if any troops had been sent in to Aden from the sea we would have confirmed it in the daily briefing // as was the practice during operation Resolve Storm. All options are open. The coalition will not spare any effort backing the resistance and achieve positive outcomes on the ground. I reiterate, no troop landings were made into Yemen."
Committing ground troops hasn't been ruled out but it would be a major escalation of the war.
Saudi-led airstrikes, and heavy artillery from the border, began pounding Houthi rebels in March.
But so far Saudi Arabia has stopped short of putting boots on the ground- conscious it would lead to a protracted and bloody ground war.
The coalition trying to restore his presidency includes eight other Arab states - some remain opposed to a ground war.
Yemen analyst Peter Salisbury says the well-armed fighters are probably Yemenis.
"It's too early to say definitively that they are Yemenis but the news that I am getting out of the south, out of the southern resistance in Aden, is that they are in fact Yemenis, that they were trained abroad for a period that they were equipped by the coalition, mainly by Saudi Arabia and that they have come in with the ability to coordinate more closely with the coalition. And with heavier weapons than have been available thus far."
Egypt and Pakistan have signalled they will not take part in a fighting force on the ground in Yemen.
But Peter Salisbury believes the fighters could be clearing a path for Saudi-led ground offensive.
"But what seems to be happening is a plan coalescing around using the Yemanis themselves to take Aden, inserting people to make sure Aden itself is secure and then using that as a base to arm and equip anti- Houthi forces // which may prove a lot more difficult than the Saudis imagine given that they thought it would be relatively easy to bring people around the ousted president Abd Rabbur Mansur Hadi after he was ejected from Yemen in march."
Heavy fighting in Aden continues, despite gains by forces loyal to President Abd Rabbur Mansur Hadi.
And Human Rights watch is accusing the Saudi-led coalition of resorting to using cluster bombs.
Far from relying on a ceasefire and negotiations for a political solution, Saudi Arabia may be preparing for a bloody ground war to stabilise Yemen.
Peace talks are scheduled for later this month in Ryadh but the Houthis and supporters of Ali Abdullah Saleh are not invited.