Syrian tanks have opened fire on poor Sunni districts in the port city of Latakia, in the fourth day of fighting which has claimed at least 34 lives, a human rights group says.

President Bashar al-Assad has broadened and intensified a military assault against towns and cities where demonstrators have been demanding his removal since the middle of March.

"Heavy machine gun fire and explosions were hitting al-Raml al-Filistini (home to Palestinian refugees) and al-Shaab this morning. This subsided and now there is the sound of intermittent tank fire," one resident, who lives near the two districts of Latakia, said.

The Syrian Revolution Coordinating Union, a grassroots activists' group, said six people, including were killed in Latakia on Monday (local time), bringing the civilian death toll there to 34, including a two-year-old girl.

The crackdown coincided with the August 1 start of the Muslim Ramadan fast, when nightly prayers became the occasion for more protests against more than four decades of Baathist party rule.

Syrian forces have already stormed Hama, scene of a 1982 massacre by the military, the eastern city of Deir al-Zor, the southern city of Deraa and several north-western towns in a province bordering Turkey.

"The regime seems intent on breaking the bones of the uprising across the country this week, but the people are not backing down. Demonstrations in Deir al-Zor are regaining momentum," one activist in the city said.

Mr Assad has been repeatedly warned by the United States, European Union and Turkey but his government is signalling to its legion of critics abroad that it will not bow to calls for change that have swept across the Arab world, and to its people that it is prepared to wade through blood to stay in power.

In Deir al-Zor, residents say the army has pulled out anti-aircraft guns from the city, but armoured personnel carriers remain at main junctions and troops, accompanied by military intelligence, storming houses looking for wanted dissidents.

Turkish foreign minister Ahmet Davutoglu has told Mr Assad to halt such military operations now or face unspecified consequences.

"This is our final word to the Syrian authorities, our first expectation is that these operations stop immediately and unconditionally," Mr Davutoglu said in Turkey's strongest warning yet to its once close ally and neighbour.

Turkish leaders, who have urged Mr Assad to end violence and pursue reforms in Syria, which has a 75 per cent Sunni majority, have grown frustrated. Mr Davutoglu held talks with the Syrian leader in Damascus only last week.

Mr Assad, who inherited power in 2000 from his father, clearly believes overwhelming force will extinguish calls for the dismantling of the police state and the Assad clan's power monopoly, free elections and an end to corruption.

For Mr Assad to enact the reforms he has been promising since he came to power in 2000, he would have to purge the Syrian establishment of his most powerful allies and cronies, and end the control of the security apparatus over the state. Since they are the foundation of his power, that is unlikely.

Reuters