Syrian troops killed 10 civilians in heavy fighting in Homs on Saturday, activists said, and one group said suspected army deserters killed 20 soldiers in the city which has become a centre of armed resistance to President Bashar al-Assad.
The Syrian Organisation for Human Rights said that the soldiers were killed in the Bab Amro district of Homs, while in the northern province of Idlib army defectors ambushed security forces, killing 10 of them.
Saturday's bloodshed came a day after activists and residents said Assad's forces shot dead 40 people when they fired on demonstrations calling for international protection from Assad's crackdown on Syria's seven-month-old uprising.
The Arab League and United Nations condemned the violence.
U.N. Secretary-General Ban Ki-moon said Assad must respond to demands for change with serious reform, not repression and violence, and called for an immediate halt to military operations, his spokesman said.
The Observatory's report of military fatalities highlights the emerging armed insurgency alongside mainly peaceful street protests demanding an end to 41 years of Assad family rule.
The United Nations says 3,000 people, including nearly 200 children, have been killed in the unrest. Since the start of protests in March, Syrian authorities have blamed the violence on gunmen they say have killed 1,100 soldiers and police.
Syria has barred most international media, making it hard to verify accounts from activists and authorities.
But the resilience of the protesters, the determination of authorities to crush dissent and the growing ranks of defectors have combined to make Syria's turmoil one of the most intractable confrontations of this year's Arab uprisings.
Activists and residents said another seven people were killed elsewhere in the country on Saturday in attacks on pro-democracy protests and in military operations hunting for defectors in rural areas.
Among them was poor labourer Salman Akko, among thousands of impoverished Kurds who fled a water crisis in eastern Syria and settled in shantytowns around Kiswa just south of Damascus.
His cousin, exiled Kurdish opposition figure Massoud Akko, said Salman was marching in one of the daily demonstrations in Kiswa when a bullet hit him in the head.
They may have not intended to kill him in particular, but security forces fired live ammunition at the protest. He lived a miserable life trying to secure daily living for his wife and kids, and now they are left with nothing, he said.
Friday's violence prompted Arab ministers to issue their strongest call yet on Assad to end the killing of civilians.
The Arab League's committee on the Syrian crisis said it had sent an urgent message ... to the Syrian government expressing its severe discontent over the continued killing of Syrian civilians.
The committee expressed the hope that the Syrian government would take action to protect civilians. Arab ministers are due to meet Syrian officials on Sunday in Qatar to press for dialogue between the government and opposition.
A source at Syria's Foreign Ministry, quoted by state media, said the Arab League statement was based on media lies and urged the committee to help restore stability in Syria instead of stirring sedition.
Opposition figures have said they could not sit down for talks with the government unless there was a halt to the killing of protesters, disappearances and mass arrests.
Three days left, and we have 220 martyrs and counting, read a placard carried by protesters near Damascus on Friday. Yes to dialogue -- after the downfall of the regime, said another in Homs.
Across the country, demonstrators called for international protection. A no-fly zone is a legitimate demand for Homs, read banners in the Khalidiya district.
NATO air forces played a central role in the overthrow of Libyan leader Muammar Gaddafi, but the Western alliance has shown no appetite to intervene in Syria.
Syria's opposition National Council has called for international protection. It has not explicitly requested military intervention, although street protesters have increasingly voiced that demand.
Assad has not used his air force against protesters and a no-fly zone would have little impact on the crackdown unless, as in Libya, pilots attacked his ground forces and military bases.
Syria, a majority Sunni Muslim nation of 20 million people, is dominated by Assad's minority Alawite sect, an offshoot of Shi'ite Islam.
Aware of potentially seismic geopolitical implications if Assad were to fall, leaders in the mostly Sunni Arab world have been cautious about criticising the Syrian president as they struggle with domestic challenges to their own rule.
Sunni ascendancy in Syria could affect Israel and shake up regional alliances. Assad strengthened ties with Shi'ite Iran while also upholding his father's policy of avoiding conflict with Israel on the occupied Golan Heights frontier.
(Additional reporting by Khaled Yacoub Oweis in Amman)