Story highlights As many as 21 people are killed nationwide, an opposition group says

Report: President al-Assad warns Western countries, defends security forces

Armed army defectors ambush Syrian security forces, says another group

The U.N. chief urges an end to operations targeting civilians

Syrian forces pounded the western city of Homs Saturday with jets and tanks, sustaining fire with machine guns and rocket-propelled grenades, activists told CNN.

At least 21 people were killed in clashes countrywide, including 11 in Homs, according to the Local Coordination Committees (LCC) of Syria, an opposition group that organizes and documents protests in Syria.

Four people were killed in Hama and one each in Hasaka, Daraa, Idlib, Keseh, Zabadany and Deir Ezzor, the LCC said.

The Syrian Observatory for Human Rights reported at least 10 civilian deaths in Homs on Saturday.

The fierce fighting has been ongoing between armed military defectors and loyalist forces, said Rami Abdel Rahman, president of the London-based group.

For weeks, the military has parked tanks at the entrance to Bab Amer, a Homs neighborhood known as the epicenter of anti-government activity, an activist told CNN Saturday. After the Saturday morning crackdown, the military established new checkpoints and a much heavier presence, the activist said.

He said he heard jets overhead around 9:30 in the morning. A few minutes later, tanks began firing into buildings and homes.

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During clashes in Bab Amer Saturday, 20 loyalist troops were killed and 53 injured, said the Syrian Observatory.

Security forces also cracked down in other Syrian cities, the LCC reported. There was heavy presence of troops in parts of Idlib and raids in Deir Ezzor.

Armed army defectors ambushed a bus carrying Syrian security forces as it traveled between two villages in Idlib province, the Syrian Observatory said. The group said 10 loyalists were killed and one defector died in clashes that broke out between the opposing groups after the ambush.

In Khan Sheikhoun, security forces opened fire on a funeral procession, the LCC said. Several soldiers defected to defend the mourners, the group said.

CNN could not independently confirm the accounts because Syria has not granted international media access to the country.

Also Saturday, the Sunday Telegraph published an interview with embattled Syrian President Bashar al-Assad, which it said was his first with a Western journalist since the start of the uprising seven months ago.

In it, the president warns Western countries not to intervene in his country.

"Syria is the hub now in this region. It is the fault line, and if you play with the ground you will cause an earthquake. ... Do you want to see another Afghanistan, or tens of Afghanistans?

"Any problem in Syria will burn the whole region. If the plan is to divide Syria, that is to divide the whole region," said al-Assad, the British newspaper reported.

While he admitted "many mistakes" had been made, the president also defended his security forces.

"We have very few police, only the army, who are trained to take on al-Qaeda," al-Assad told the Sunday Telegraph. "If you sent in your army to the streets, the same thing would happen. Now, we are only fighting terrorists. That's why the fighting is becoming much less."

Saturday's clashes erupted after Friday's massive demonstration in Hama where Syrians demanded an end to al-Assad's rule and called for his prosecution.

Demonstrators also called on the United Nations to impose a no-fly zone, naval blockade and other measures to protect Syrian protesters.

Security forces fatally shot 35 civilians during protests across the nation Friday, the Syrian Observatory for Human Rights said. At least 100 others were wounded and 500 arrested in several provinces, the group said.

U.N. Secretary-General Ban Ki-moon condemned Friday's killings. He said the deaths of more than 3,000 people killed since the start of the Syrian uprising amounts to an "alarming" toll.

Ban appealed for an end to military operations against civilians.

Meanwhile, the state-run news agency SANA reported that funerals were held Saturday for 15 members of the security forces killed in recent clashes.