Houthi rebels and their allies in Yemen have stormed the presidential palace in Aden following heavy clashes, officials say.
The rebels pushed through to the heart of the port city using tanks and armoured vehicles, despite air strikes by a Saudi-led coalition.
At least 44 people have been killed in the violence, including 18 civilians, according to AFP.
Separately, a Saudi border guard has reportedly been killed by gunfire.
It is the first known Saudi fatality since it began an air campaign against the Houthis last week.
The Saudi government says the aim of its operation in Yemen is to protect President Abdrabbuh Mansour Hadi's "legitimate government". It says it has no plans to deploy ground forces for now.
Aden was President Hadi's last seat of power before he fled to Saudi Arabia last week, as the Shia rebels advanced on the city.
"There are bodies and wounded in the streets and nobody dares to approach," said a resident in the central neighbourhood of Crater.
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
Earlier reports suggested that foreign troops had disembarked at the port. But China later said it had sent its navy to the area to help foreign nationals evacuate.
The Houthis have said their aim is to replace President Hadi's government, which they accuse of being corrupt. They captured the president's palace in the capital Sanaa in January and established a transitional presidential council.
Gulf Arab states have accused Iran of backing the Houthis financially and militarily, though Iran has denied this.
Fighting in Yemen over the past two weeks has left more than 500 people have dead and some 1,700 wounded, UN humanitarian chief Valerie Amos said.
'We just want peace' - eyewitness Nisma in Aden
It's not safe to go out; it's not safe to go anywhere.
We fled from our home yesterday. The Houthis were bombing everywhere - gunshots, RPGs, everything you can imagine.
When President Hadi came to Aden, he just brought all the mess from Sanaa.
In Sanaa they have weapons: they could have fought the Houthis. We only have light weapons. Can you imagine fighting a tank with a gun? It's insanity.
Hadi does not deserve to be fought for. Aden is a peaceful city, Adeni people don't carry weapons.
After two days of bombing, he flew away to Saudi Arabia, and now people are dying here and he is safe and doing nothing for us. He hasn't even said thank you to his people for protecting him and Yemen.
We are grateful to Saudi Arabia but the air strikes are not effective anymore. I'm worried about what will happen. An invasion could turn us into Iraq. We don't want airstrikes, we don't want invasions, we just want peace.
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.
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.
Local Yemeni officials said that among those released in the prison break was a senior figure in the regional wing of al-Qaeda.
He was named as Khaled Batarfi, who had been arrested four years ago.