Troops loyal to Muammar Gaddafi have been seen outside the besieged Libyan city of Misurata wearing gas masks, according to several sources on the ground, raising the prospect of a gas or chemical attack.

Different eyewitness and sources in touch with civilians told The Times they saw troops wearing gas masks a day after a Nato air strike killed Col Gaddafi’s youngest son and three grandchildren in Tripoli.

Meanwhile, the UN withdrew its international staff following a mob attack on its offices, while the UK expelled the Libyan Ambassador to London following attacks on its evacuated embassy premises in Tripoli. The Times received reports that other vacant embassy properties, including that of the US and Italy, came under attack but this could not be confirmed.

Misurata was reported to be under heavy, incessant shelling since 3 p.m. on Saturday local time.

Tanks have been positioned around 35 kilometres to the east, south and west of the rebel-held city.

A man stationed in one of the control rooms set up by the Libyan transitional council there said people had witnessed pro-Gaddafi forces roaming the streets of Zlitan on their way to Misurata wearing gas masks under the command of the dictator’s second eldest son, Moutasim Gaddafi.

Moreover, he said a few days ago, the rebels discovered a stock of chemical weapons when they won over the Air Force academy in Misurata

A source in touch with civilians on the ground said: “The winds are blowing from the east right now which would make a gas attack unlikely but many people are viewing the death of Gaddafi’s son (Saif al-Arab) as the tipping point which could trigger the worst feared retaliation.”

A Libyan man from Misurata echoed these comments, saying civilians in the city had been warned about a possible gas attack and have gathered at each others’ houses and in public buildings like schools.

The regime is attacking the city’s centre and not military bases, he said, adding that Misurata was not prepared for such an attack.

“People are already having to make ends meet with whatlittle they have, and even though this is an evidently unfair game, Gaddafi will not think twice about killing innocent civilians,” he added.

Back in Tripoli, another witness said embassy buildings had been broken into, robbed and set on fire, in the presence of police officers and soldiers.

“Tripoli is unrecognisable,” he said. “It’s scary. We meet mercenaries roaming streets every day. Sub-Saharans have been stationed at checkpoints in the capital city. We communicate through sign language.

“Soldiers have been grouped in packs of 700, while armed canons are stationed at every corner.

“Most of the houses are devoid of men, and women have not set foot outside of their homes for weeks,” he added.

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