Syrian government troops fought fierce battles with rebels on Wednesday for control of key neighbourhoods in the north of Damascus, President Bashar al-Assad's seat of power, residents and activists said.

Also in the Syrian capital, a European Union staff member was killed in a rocket attack in an opposition stronghold south of the capital, the EU said.

Opposition fighters trying to topple Assad have been trying to take Damascus for weeks, battering regime checkpoints and military bases in the heavily fortified capital. They have also fired mortars into residential districts and into the capital's main football stadium, sowing fear among residents.

Both sides see Damascus as the ultimate prize in the civil war.

The Syrian Observatory for Human Rights said Wednesday's clashes were concentrated in the capital's neighbourhoods of Jobar and Barzeh.

A resident in the area said shelling overnight "shook apartments" and terrified the inhabitants. He spoke on condition of anonymity because he was afraid for his safety.

Clashes subsided by early Wednesday but sporadic gunfire could still be heard in the contested districts.

A car bomb exploded outside a police station in Khan Sheih neighbourhood, west of Damascus, the Observatory said. The Britain-based activist group also said fierce clashes broke out after the blast but had no immediate reports of casualties.

Fighting also raged in other Syrian cities, including in Homs, where the regime pounded rebel positions with artillery and carried out several airstrikes on the Baba Amr district, a former rebel stronghold which the opposition has tried to recapture in the past days.

1st EU employee killed

In Brussels, the EU foreign policy chief Catherine Ashton said a policy officer with the European delegation in Syria was killed on Tuesday in the Damascus suburb of Daraya. It was the first death in the Syrian civil war of an EU employee.

Ahmad Shihadeh, 32, had worked for the EU for five years, a spokesman for Ashton said Wednesday. He said Shihadeh had lived in Daraya, a suburb of Damascus that has been one of the main battlefields in the capital.

Ashton said he "died while providing humanitarian help to the community of Daraya," Ashton said. "Ahmad was known for his courage and selflessness."

Ashton took the occasion to call for an end to the conflict, which started in March 2011 as peaceful protests against Assad's authoritarian rule. The revolt turned into civil war after some opposition supporters took up arms to fight back against a harsh government crackdown on dissent.

"As we approach the second anniversary of the uprising in Syria, I call again on all sides to take urgent steps to end the violence, which has led to the deaths of some 100,000 innocent citizens and over one million refugees seeking shelter in neighbouring countries," she said.

More than 70,000 people have been killed in the 2-year-old conflict and four million Syrians were driven from their homes by the fighting, according to United Nations' assessment.