SANAA (AFP) - Dozens of civilians were reported dead on Wednesday after a dairy plant was bombed in Yemen, as aid groups warned of a brewing humanitarian crisis from the Saudi-led coalition's strikes on Shi'ite rebels.

Yemeni Foreign Minister Riyadh Yassin called for the coalition to send in ground troops, saying that, "at some stage, air strikes will be ineffective".

Diplomats said meanwhile that Gulf countries were locked in tough negotiations with Russia on a UN draft resolution to impose an arms embargo and sanctions on the Huthi Shi'ite rebels.

But rights groups have voiced growing alarm about civilian casualties from the nearly week-old air war aimed at preventing the fall of President Abedrabbo Mansour Hadi.

At least 37 workers were killed and 80 wounded overnight at the dairy in the port of Hodeida, Governor Hasan al-Hai said, without specifying whether the factory was hit by an air strike or rebel shelling.

The provincial health chief, Abdulrahman Jarallah, gave a toll of 35 people killed and dozens wounded.

Part of the factory was destroyed and rescue teams were looking for survivors under the rubble, according to a medic.

The circumstances of the bombing were unclear, with some witnesses saying the dairy was hit by a coalition strike and others blaming rebels loyal to former president Ali Abdullah Saleh.

Coalition spokesman Brigadier General Ahmed Assiri accused the Huthis of targeting the factory, using "mortar shells and Katyusha rockets... in a bid to create unrest among the Yemeni society".

The Arab forces would continue to "evaluate targets" and "lower the chances of targeting civilians or aid workers," he said, urging humanitarian organisations to "contact concerned parties in order to facilitate this process."

Assiri said the latest Saudi-led operations targeted rebel brigades in Daleh, Aden and Shabwa, all provinces in Yemen's south.

Pro-government forces had seized the town of Daleh, and a bid to also take Shabwa was showing "positive" results, he added, urging the renegade troops to rejoin the forces of the "legitimate government".

The coalition bombarded rebel positions in the main southern port city Aden in a seventh night of raids that also targeted the capital and other areas.

Those strikes focused on the rebel-held provincial administration complex in Dar Saad in the city's north, said a military official.

Wednesday's clashes between rebel forces on one side, and residents and local militia on the other, sparked by the Huthi advance on Aden's Khor Maksar district killed at least 19 people, six of them civilians, officials said.

FRESH STRIKES IN CAPITAL

The coalition has vowed to keep targeting the Huthis and allied army units loyal to Saleh until they end their insurrection.

Egyptian President Abdel Fattah al-Sisi urged the rebels to "back off" for the sake of their country.

Iran is accused of backing the rebels but Teheran denies providing military support.

Air strikes targeting military bases in Sanaa resumed later Wednesday as rebel air defence systems fired back, residents said.

A medic from the University Of Science & Technology Hospital said a mortar landed at the facility's entrance, wounding five civilians.

Six civilians were killed in an air raid targeting Maydi in the north-west province of Hajjah, medics said.

After entering the capital in September, the Huthis and their allies gradually conquered areas in the centre, west and south of Yemen before bearing down on Aden last month, prompting Hadi to flee to Saudi Arabia.

The embattled president had taken refuge in Aden in February after escaping house arrest in the capital.

'VERGE OF COLLAPSE'

Since Friday, at least 93 civilians have been killed and 364 wounded, the UN said Tuesday.

"We have reports that the hospitals are really full of dead and injured people," spokeswoman Cecile Pouilly told AFP.

"We have heard about lots of dead bodies."

UN High Commissioner for Human Rights Zeid Ra'ad Al Hussein warned that Yemen "seems to be on the verge of total collapse".

Foreign Minister Yassin said sending ground troops would cause "less civilian casualties" and enable aid deliveries.

A Western diplomat said a land offensive would be complicated because it would have to pass mountainous terrain in the country's north, with which the Huthis are highly familiar.