Syria - The Syrian army has resumed shelling in Homs killing at least 13 people so far on Thursday, activists say.

With several districts controlled by rebel forces, Syria's third-largest city is a major focus of unrest against President Bashar al-Assad's rule.

Scores have been killed since the army started an assault there last week.

UN chief Ban Ki-moon says said the failure to agree a UN resolution on Syria had encouraged Damascus "to step up its war on its own people".

The international community is struggling to find a way to resolve the crisis after Russia and China blocked a text drafted by Arab and European countries last week.

Activists say those killed as bombardments resumed early on Thursday include three whole families.

On Wednesday witnesses in the Baba Amr district reported intense shelling by tanks, mortars, artillery and heavy machine guns. More than 50 people died, according to the UK-based Syrian Observatory for Human Rights.

"The situation is dire. We are short of food, water and medical aid. Doctors have collapsed after treating the wounded without rest for five days," said Homs resident Omar Shaker.

Syria restricts access to foreign media and casualty figures cannot be independently verified.

The army says it is fighting foreign-backed armed groups. Army defectors have joined rebel forces in Homs and other parts of Syria in recent months.