Houthi rebels in Yemen have intensified their push to control the port city of Aden, amid reports of troops landing from the sea.
Initial reports suggested the troops were foreign but an adviser to the Saudis, who are leading a coalition against the Houthis, denied this.
He said they could be loyal to deposed president, Ali Abdullah Saleh, who backs the rebels.
Separately, a Saudi border soldier has reportedly been killed by gunfire.
It is the first known Saudi fatality since the coalition began its air campaign against the Houthis.
Also on Thursday, local officials in the port city of Mukalla said al-Qaeda militants had stormed a prison there and freed at least 150 detainees.
Houthi rebels have been trying to advance into the heart of Aden using tanks and armoured vehicles.
Last week they seemed close to taking the city, forcing President Abdrabbuh Mansour Hadi to flee the country.
President Hadi had previously taken refuge in Aden after the Houthis took full control of the capital Sanaa in January and placed him under house arrest.
The Houthis: Zaidi Shia-led rebels from the north, who seized control of Sanaa last year and have since been expanding their control
President Hadi: Fled to Saudi Arabia after rebel forces advanced on his stronghold in the southern city of Aden
Al-Qaeda in the Arabian Peninsula: Seen by the US as the most dangerous offshoot of al-Qaeda, AQAP opposes both the Houthis and President Hadi.
Islamic State: A Yemeni affiliate of IS has recently emerged, which seeks to eclipse AQAP
There was confusion about the nationality of the troops who landed in Aden.
The Saudi adviser told the BBC that no Saudi or coalition forces had landed in the port and that those seen by witnesses disembarking were troops loyal to Mr Saleh.
The Houthis have said their aim is to replace President Hadi’s government, which they accuse of being corrupt.
Gulf Arab states have accused Iran of backing the Houthis financially and militarily, though Iran has denied this.
The Saudi border guard killed was named by state news agency SPA as Cpl Ali Yahya al-Maliki.
Ten others were wounded when his observation post in the province of Asir came under fire.
Local Yemeni officials said that among those freed in the prison break in Mukalla was a senior figure in the regional wing of al-Qaeda.
He was named as Khaled Batarfi, who had been arrested four years ago.