UPDATE 17-Little let-up in Khartoum fighting despite Sudan truce declaration

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Countries unable to evacuate foreign citizens

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Army, RSF say they agree to three-day truce

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WHO says more than 400 people killed

(Adds RSF leader's statement in paragraph 8, changes dateline)

By Khalid Abdelaziz and Nafisa Eltahir

KHARTOUM, April 22 (Reuters) - Sporadic shelling rang out late on Friday in Sudan's capital even though warring factions announced a truce, while one force said it was willing to allow airports to reopen for the evacuation of foreign nationals.

The United Nations, U.S., UK, Japan, Switzerland, South Korea, Sweden and Spain have said they were making preparations or attempting to remove their personnel after almost a week of violence.

Forces commanded by two previously allied leaders of Sudan's ruling council began a violent power struggle last weekend. Hundreds have died, and a nation reliant on food aid has been tipped into what the United Nations calls a humanitarian catastrophe.

Artillery fire continued in Khartoum late on Friday, a Reuters witness said, though less intense than earlier in the day. The fighting dealt the latest blow to international attempts to end the fighting.

The army and its adversary, the paramilitary Rapid Support Forces (RSF), said separately they agreed to a three-day truce to enable people to celebrate the Muslim holiday of Eid al-Fitr.

"The armed forces hope that the rebels will abide by all the requirements of the truce and stop any military moves that would obstruct it," an army statement said.

U.S. Secretary of State Antony Blinken urged the combatants to honour the truce, and said Sudan's military and civilian leadership must urgently start negotiations on a sustainable ceasefire to prevent further damage to the country.

RSF chief Mohamed Hamdan Dagalo, better known as Hemedti, said early on Saturday that he had received a phone call from U.N. Secretary General Antonio Guterres. The two "emphasised the necessity of adhering to a complete ceasefire and providing protection for humanitarian and medical workers, especially U.N. staff as well as regional and international organisations," Hemedti said in a post on his official Facebook account.

The RSF said late on Friday it was ready to partially open all of Sudan's airports so foreign governments could evacuate their nationals.

The group said in a statement it would "cooperate, coordinate and provide all facilities that enable expatriates and missions to leave the country safely."

It was unclear to what extent the RSF controls Sudan's airports. The Khartoum airport has been caught in the fighting with aircraft burning on the tarmac, and commercial airlines halted flights several days ago.