Syrian government forces and the armed opposition are fighting in two Damascus suburbs, while there were also reports of overnight shelling in the capital.
The clashes have been happening in the suburbs of Qudsaya and Hammah, the Britain-based Syrian Observatory for Human Rights reports.
AP also says that fighting raged near the housing compounds and bases of the Republican Guard.
"Artillery fire can be heard regularly in the capital, but it's the first time that there’s been the sound of such heavy shelling. There was no sound of aircraft however," says RT’s Maria Finoshina, who is currently in Damascus. “There was a feeling of an operation with heavy weaponry involved. А while ago we saw helicopters flying over Damascus, and we have been hearing about fighter jets also taking part in operations. The one that that defected to Jordan, a MiG-21, was not on training but on military duty, some sources say here.”
She added that the shelling also appears to be coming from one of the presidential compounds of Bashar al-Assad, which contains the house where his father used to stay, though Bashar himself does not stay there. The area is usually known for its very high security.
Many ambulances have been reported to be rushing injured people to hospital.
The intensified clashes may indicate that a coordinated offensive may be taking place in Syria, reports Israel's Debkafile website. The attacks are apparently targeting the presidential palace and the compound of the presidential guard, the site reports.
The site says there are unconfirmed reports that British commando forces crossed from Turkey into Syria on Tuesday, according to Gulf military sources. The force may be involved in securing a safe heaven for rebel forces along the border.
There were similar reports in February about the alleged presence of British troops operating in Syria. At the time the British government denied carrying out a clandestine operation in the country.