Syria's violence has escalated sharply, with up to 28 people reportedly killed across the country on Wednesday as France bluntly accused Bashar al-Assad's regime of committing crimes against humanity.

Horrifying film clips were posted on YouTube, one showing a dead or injured man in the central city of Homs apparently being shot by uniformed men. Another showed a young man named Abdel-Hadi Mustafa dying after being shot by a sniper. A third recorded the death of 15-year-old Zakaria Firzat from al-Rastan on Tuesday. None of the incidents could be independently authenticated as most foreign journalists have been banned from Syria. Casualty figures cannot be easily verified either.

Medical sources in Homs reported patients being kidnapped from al-Birr hospital by security personnel. "We have no idea where they've taken them," said a journalist named Fadi, describing ambulances commandeered by regime forces.

Witnesses also reported an attack by the "free Syrian army" on a state security office. Activists claimed three officers and 30 soldiers had defected to the opposition. Social media described powerful explosions and smoke rising across the city.

Many of the dead were from Homs, a hotbed of opposition to the regime, and several from nearby Hama. Two were shot dead during raids in Sarameen in the north. The Syrian Revolution General Commission listed 28 dead in all.

Syrian state media highlighted a report on the funerals of 13 "martyred" soldiers and defence ministry employees. It did not explain how they had died.

Amid the bloodshed, Syria rebuffed Arab concerns about repression by putting off a visit by the head of the Arab League to discuss reforms to end the crisis. Nabil al-Arabi, secretary general of the league, had been in due in Damascus on Wednesday but was asked to postpone the trip by the government for what official media described as "substantive" reasons. Al-Arabiya TV reported that the visit would now go ahead on Saturday.

Other Arab governments broke months of silence at a meeting in Cairo last week, demanding Syria stop the bloodshed and Assad undertake political and economic reforms. Syria hit back at the league for "unacceptable and biased language".

Arabi had prepared a 13-point plan, reportedly drafted by Qatar, under which Assad would cease military operations, free all political prisoners, begin dialogue and announce his intention to form a national unity government and hold pluralistic presidential elections by 2014. Al-Ba'ath, a mouthpiece for the regime, warned that the plan was designed to turn the pan-Arab organisation into "the political arm of Nato military adventures instead of being the home of Arabs".

A league spokesman, Mutaz Salah al-Din, said its foreign ministers would discuss the Syrian crisis at a special session next week. Arab governments, discomfited by the Nato intervention in Libya, only began to speak out about Syria last month after King Adbullah of Saudi Arabia denounced Assad's "killing machine".

Opposition activists reported tanks and troops deployed in Homs. "All through the night there was shooting. The gunfire didn't stop," a resident told the Associated Press. "I can't tell exactly what is going on because it's dangerous to go out."

France, meanwhile, ratcheted up the pressure on Assad. "The Syrian regime has committed crimes against humanity," its foreign minister, Alain Juppé, said during a meeting with his Russian counterpart, Sergei Lavrov.

The UN believes 2,200 people have been killed in the uprising. Human rights groups say that more than 10,000 Syrians are currently in prison.

The US and other western governments have hinted that they are considering referring Syrian officials to the international criminal court, following the example of Libya, but admit privately that there is not enough UN security council support for such a referral.

On Tuesday Ban Ki-moon, the UN secretary general, said during a visit to New Zealand that Assad must take "bold and decisive measures before it's too late".

France has said the EU is preparing a new round of sanctions against Syria that would target economic entities after the imposition of a ban on purchases of Syrian oil last week. This is expected to hit the government hard as the EU buys 95% of Syria's crude exports, providing a third of its hard currency earnings.

• This article was amended on 8 September 2011. The original described Ban Ki-moon as the US secretary general. This has been corrected.