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Up to 20 aircraft, said to include Iraqi Migs and Mirages, were seen overhead at around 1100 local time in Halabja.
According to experts, the chemicals dropped by the planes may have included mustard gas, the nerve agents sarin, tabun and VX and possibly cyanide.
The attack on Halabja, which is about 150 miles (241km) north-east of the Iraqi capital Baghdad, is the latest in the Iran-Iraq war and follows its occupation by Iranian forces.
Iraq was said to be keen to avenge the fall of Halabja, which is seen as an important centre for Kurdish resistance in their struggle for autonomy.
The assault came after two days of conventional mortars, artillery and rockets from nearby mountains.
According to pro-Iranian Kurdish commanders in Halabja, there were up to 14 aircraft sorties, with seven to eight planes in each group.
The planes were believed to have concentrated their attacks on the city and all the roads leading out of it.
Eyewitnesses have told of clouds of smoke billowing upward "white, black and then yellow"', rising as a column about 150 feet (46 metres)in the air.
Most of the wounded, who were taken to hospital in the Iranian capital Tehran, were suffering from mustard gas exposure.
Those who escaped death have developed respiratory or visual problems from the cocktail of chemicals dropped on the city.
According to some reports, up to 75% of the victims were women and children.
The injured survivors seen by reporters showed the classic symptoms of mustard gas poisoning - ugly skin lesions and breathing difficulties.
Some residents survived by covering their faces with damp cloths and taking to the mountains around Halabja.
One resident, Abdul Rahman, 60, an employee at the city's mosque, said: "I do not know where my children are."
Most of the details about the Halabja killings only emerged a few days later. Reports from the city suggested that Iraqi leader Saddam Hussein's forces had launched the chemical gas attack.
Figures for the final total of dead range from 3,200 people to 5,000.
Between 7,000 and 10,000 are believed to have been injured in the massacre, which became known as "Bloody Friday".
Initially, the US Defence Intelligence Agency blamed Iran for the attack. Halabja is around eight to 10 miles (14km to 16km) from the Iranian border.
However, the majority of evidence indicates that the gas attack was an Iraqi assault against Iranian forces, pro-Iranian Kurdish forces and Halabja's citizens during a major battle.
Although there is some evidence Saddam Hussein's forces had used chemical agents before this date, the attack on Halabja is thought to be the first documented assault using chemicals.
Saddam Hussein's deputy - Ali Hassan al-Majid, or "Chemical Ali" - who is on trial charged with crimes against humanity over a campaign against Kurds in the 1980s
He is one of six defendants facing charges of war crimes and crimes against humanity during the so-called Anfal campaign that killed an estimated 100,000 people.
The tribunal dropped charges against the seventh co-defendant, Saddam Hussein himself, when he was executed on 30 December 2006 after being convicted in a separate case.
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