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Vahid Salemi, Associated Press
Yemeni men chant slogans while holding posters of Shiite Houthi rebel leaders as scores of Yemeni expatriates, including students and clerics, protest against Saudi-led airstrikes on their homeland, in front of the Saudi Embassy, in Tehran, Iran, Wednesday, April 1, 2015.

SANAA, Yemen — Saudi-led coalition warplanes bombed Shiite rebel positions in both north and south Yemen early Wednesday, setting off explosions and drawing return fire from anti-aircraft guns. Airstrikes also hit a factory in a western port city and 35 workers died there, according to witnesses and officials.

The bombings came as the Saudi-led campaign against Yemen's Shiite rebels known as Houthis entered its seventh day. The campaign aims to weaken the Iran-backed rebels who are also allied with forces loyal to Yemen's deposed president, Ali Abdullah Saleh and halt their power grab.

Since their advance began last year, the Houthis have overrun the Yemeni capital, Sanaa, and several provinces, and have forced the current president, Abed Rabbo Mansour Hadi, to flee abroad from the southern port city of Aden.

Wednesday's airstrikes targeted rebel-controlled army camps in the Red Sea port city of Hodeida, and hit warehouses belonging to a factory that produces dairy products.

Anti-aircraft guns returned fire before the factory itself was set ablaze. Parts of its main building collapsed from explosions with workers still inside, according to at least five eyewitnesses and officials. According to the medical center in Hodeida, at least 35 workers were killed, many of them crushed by the rubble or burned to death.

Two military officials said the factory had been used as a rebel weapons cache, and that while the airstrikes flattened the warehouses, the main factory building was only partially destroyed — suggesting it might have been hit from ground. All the officials and witnesses spoke on condition of anonymity because they were not authorized to speak to the media.

The Hodeida death toll came a day after international aid groups expressed alarm over high civilian casualties in Yemen's escalating crisis.

A report Tuesday by the U.N. children's agency, UNICEF, said 62 children were killed and 30 wounded during the fighting in Yemen over the past week. It was not clear if the deaths were the result of airstrikes or ongoing clashes between rival groups across the country.

Also Tuesday, the U.N. human rights office in Geneva said its staffers in Yemen confirmed that at least 19 civilians died when airstrikes hit a refugee camp near the Houthi stronghold of Saada in northern Yemen on Sunday, with at least 35 wounded, including 11 children.

Critics of the Houthis charge that they are an Iranian proxy — a claim the rebels deny. Iran has provided aid to the rebels, but both Tehran and the Houthis deny it has armed them.

In Tehran, dozens of Yemeni expatriates, including clerics and students, took to the streets on Wednesday to denounce the airstrikes. The protesters burned photos of Saudi King Salman and marched outside the Saudi Embassy.