(CNN) -- At least 21 people were killed in a Syrian military crackdown on anti-government protesters in the northwestern port city of Latakia Sunday, opposition groups said.

Tens of people were injured when naval forces shelled the city Sunday and security forces opened fire in residential neighborhoods, according to the Syrian Observatory for Human Rights.

The Local Coordination Committees of Syria, a network of opposition activists operating from inside Syria, said that at least 25 people were killed, including three children.

The crackdown in Latakia is the latest in a series of military actions to target protesters calling for the ouster of President Bashar al-Assad.

Syrian gunboats shelled at least one neighborhood in Latakia, said Rami Abdel Rahman, the London-based observatory's president. Forces were using medium-range weapons and machine guns in some neighborhoods, according to the observatory group and the Local Coordination Committees.

"These are small residential neighborhoods so mass organized resistance is difficult. Some people are trying to protect their homes but the streets are dominated by Syrian security forces and their weapons," Rahman said.

The opposition groups attributed the accounts to eyewitness reports from residents and medical professionals, whom they did not identify by name because of security concerns. They said two people were killed in clashes Saturday.

CNN cannot independently verify the reports because the government has restricted international journalists from reporting in Syria. The Syrian government did not immediately respond to a CNN request for comment.

Latakia has been a flashpoint in the Syrian uprising that began in March with mass protests following the arrest of teens for scrawling anti-government graffiti.

Protesters have demanded free elections and an end to the reign of al-Assad, whose Alawite-minority led government rules Syria.

In March, at least a dozen people were killed when security forces clashed with protesters. Among the dead were civilians and members of the security forces, according to a report by government at the time.

The fighting in Latakia comes as international pressure mounts against al-Assad to end the brutal crackdown, which he has said is the result of armed gangs attacking residents as well as security forces.

CNN's Yousuf Basil, Amir Ahmed, Arwa Damon, Nada Husseini, Yesim Comert and Salma Abdelaziz contributed to this report.