(CNN) -- Syrian troops rolled into the northeastern city of Deir Ezzor with tanks and bulldozers early Sunday morning, part of a renewed crackdown on anti-government protests, an opposition activist told CNN.
The latest attack by Syria forces followed a call a day earlier by U.N. Secretary-General Ban Ki-moon that Syrian President Bashar al-Assad halt the use of force on civilians "immediately."
Ban spoke to the Syrian leader by phone Saturday to express "his strong concern and that of the international community at the mounting violence and death toll" in the country over the past week, according to a U.N. readout of the conversation.
The conversation took place after anti-government protests erupted Friday across the country, the latest round of demonstrations calling for reforms and an end to al-Assad's reign.
The protests began months ago in the southern city of Daraa and were swiftly suppressed by Syrian forces. Anti-government fervor caught on nationwide as more protests were met with tougher crackdowns.
Loud explosions could be heard in a number of neighborhoods in Deir Ezzor after troops entered the city in northeast Syria on Sunday, an opposition activist said.
The activist, who CNN is not identifying for security reasons, said residents in Deir Ezzor have put up barricades made up of poles and rocks to try to block an assault.
The military action follows the posting of a video Friday on YouTube that showed a man warning tribal leaders that Syrian forces would storm the city. In the video, the man tells tribal leaders to prepare for a military assault.
Over the weekend, clashes between Syrian forces and demonstrators were reported in a number of cities, including Daraa and Idlib, witnesses said.
In the Syrian capital of Damascus, heavy gunfire could be heard in the suburb of Nahr Aysha where security forces were trying to disperse thousands of demonstrators, the witnesses said.
Syrian forces also shelled the western city of Hama, another flashpoint in the months-long Syrian uprising, said a resident, whose identity is being withheld by CNN for security reasons.
More than 50 people were killed when a tank positioned 150 meters (164 yards) from Hama's Hurani Hospital launched an artillery shell that landed in front of the building, two witnesses told CNN. One witness said he counted 53 bodies; the other said 58.
Another opposition activist estimated 300 people have died in fighting in Hama in the past six days.
The Syrian Observatory for Human Rights, an activist group, said 2,000 people have died in the unrest, mostly demonstrators.
In his conversation with the U.N. leader, Al-Assad pointed to reform measures announced this week under international pressure to end the crackdown on peaceful protests. The measures include a decree authorizing a multiple-party political system.
Ban "underscored that for these measures to gain credibility, the use of force and mass arrests must stop immediately," the U.N. statement said.
Meanwhile, the country's foreign minister announced plans Saturday to hold parliamentary elections by the end of the year, the state-run news agency reported.
Walid Moallem told Arab and foreign ambassadors that the country's leadership is committed to move ahead with reforms and provide free and fair elections for the country, the Syrian Arab News Agency reported.
Moallem said Syria is intent on fostering security, stopping vandalism, and pursuing democracy and progress, SANA reported.
CNN's Kamal Ghattas contributed to this report.