A statement early yesterday by the Syrian National Coalition, an umbrella group of opposition political forces, said it also had suspended participation in a Friends of Syria conference of international powers due in Rome next month to protest over the attacks it said have caused many civilian casualties.
"Hundreds or civilians have been killed by Scud missile strikes. Aleppo, the city and the civilisation, is being destroyed systematically," the statement said.
"The Russian leadership especially bears moral and political responsibility for supplying the regime with weapons," it added, referring to Moscow's status as a leading ally of Syrian president Bashar al-Assad. "In protest of this shameful international stand, the coalition has decided to suspend its participation in the Rome conference for the Friends of Syria and decline the invitations to visit Russia and the US."
The invitations had been extended to opposition coalition leader Mouaz al-Khatib after he met the Russian and American foreign ministers in Munich earlier this month.
The invitations were made shortly after al-Khatib offered to negotiate Assad's departure with those members of the Syrian government not tainted by having participated in the crackdown on the 23-month-long revolt.
Rocket attacks on eastern districts of Aleppo, Syria's industrial and commercial hub, killed at least 29 people on Friday and trapped a family of 10 in the ruins of their home, opposition activists in the city said.
Meanwhile, in neighbouring Turkey, Germany's defence minister yesterday inspected Patriot missile batteries close to the Syria-Turkey border and said they delivered a "clear warning" to Damascus that Nato would not tolerate missiles being fired into Turkey.
Thomas de Maiziere and his Dutch counterpart, Jeanine Hennis-Plasschaert, travelled to the Turkish cities of Adana and Kahramanmaras to inspect the batteries provided by their countries at Turkey's request. German Chancellor Angela Merkel was scheduled to visit the same area today when she begins a two-day visit to Turkey.
The US has also sent Patriots, which are capable of shooting down hostile missiles in mid-air.
"Our presence here serves to make sure that Syria doesn't turn its capabilities into action," de Maiziere said. "We can see from here that Syria is using rockets – often several times a day."
Syria is believed to have more than 1000 rockets with a range of up to 700km, and around 1000 tonnes of chemical weapons material.
Turkey is a staunch supporter of the uprising against Assad and has harboured both Syrian refugees and rebels. Violence has sometimes spilled over the border.
Tensions have increased in recent weeks after Nato said it had detected launches of short-range ballistic missiles inside Syria, several of which have landed close to the Turkish border. Turkey has scrambled war planes along the frontier, fanning fears the war could spread and further destabilise the region.
"The Patriot system is strictly for defence, and placing them on our soil within the Nato framework was to protect our people and our soil against possible attack," said Turkish defence minister Ismet Yilmaz.