Thousands of Yemeni civilians are trapped on the southern outskirts of the Red Sea port of Hodeidah as forces backed by a Saudi-led coalition battle Houthi fighters entrenched in the city, aid groups said.

The alliance has massed thousands of Yemeni troops in recent days near the heavily defended port despite recent calls from international leaders for an end to the country's ongoing conflict, which has unleashed the world's "worst humanitarian crisis".

Rebels and government officials reported intense battles on Monday near the western port city, according to AFP news agency.

"All the people living between the airport and the university are trapped, the last four days have been very tough, it is beyond catastrophic levels," Isaac Ooko, Hodeidah area manager for the Norwegian Refugee Council, told Reuters news agency.

"Air strikes have been very intense and the hovering of the jets causes permanent anxiety ... Hodeidah has become a ghost city, people stay indoors and the streets are deserted."

Yemeni military officials told AFP that government forces backed by the US-supported Saudi-UAE military coalition advanced on Houthi-held Hodeidah and positioned themselves around both the north and south of the city in a bid to surround it and block a major rebel supply route.