- NEW: At least 21 people are killed Sunday, an opposition group says
- The conflict also heightens tensions in Lebanon, where a soldier and a civilian are killed Sunday
- A video purportedly from a terror group claims responsibility for twin Damascus blasts
- A United Nations observer mission has monitors in several Syrian provinces
Monday night, Anderson Cooper reports live from the Syria/Turkey border on the turmoil in the country. Watch "Anderson Cooper 360°" at 8 ET.
(CNN) -- Terror gripped cities across Syria on Sunday as residents came under attack by shelling and gunfire, opposition activists said.
Many homes caught fire as Syrian forces stormed Hama, the opposition Syrian Observatory for Human Rights said. At least eight people were killed in the western city, according to Local Coordination Committees of Syria, an opposition network.
Explosions were also heard near the southern city of Douma and near a government security branch in the eastern Deir Ezzor province, the observatory said.
The Syrian conflict has also heightened historic tensions across the border in Tripoli, Lebanon, where at least one Lebanese civilian and one Lebanese soldier were killed Sunday in clashes between factions supporting and opposing the uprising in Syria, Lebanon's National News Agency reported.
The fighting in Lebanon took place between residents of Bab al-Tebbaneh, a Sunni stronghold hostile to the Syrian regime, and Alawite residents of Jabal Mohsen who support Syrian President Bashar al-Assad.
Inside Syria's borders, the crisis is getting more complicated.
A video purportedly released by a shadowy, Syrian-based terrorist group claimed responsibility Saturday for dual suicide bombings that killed dozens and wounded hundreds in the country's capital last week.
The two-minute video, apparently by jihadist group Al Nusra Front, says government buildings were targeted "because the regime continues to shell residential civilians in Damascus countryside, Idlib, Hama, Daraa and other areas. And we remained true to our promise to respond to this shelling with strikes."
Syrian Prime Minister Adel Safar visited the site of the bombings in Damascus on Saturday, state-run media reported.
The suicide attacks were aimed "at striking Syria's will and its people in life, security and stability," Safar said, according to the official Syrian Arab News Agency.
The attacks Thursday killed at least 55 people. The government has blamed "terrorists," while members of the Syrian opposition have blamed the government.
Since the bombings, reports of bloodshed only continue to mount.
Syria releases journalists The Local Coordination Committees of Syria reported at least 21 deaths on Sunday, including eight in Hama, three in Homs, three in Daraa, two in Deir Ezzor, two in Idlib, two in the Damascus suburbs and one in Aleppo.
Damascus and Aleppo have been the scene of a flurry of attacks in recent months. Aleppo, a commercial center and long a bastion of support for al-Assad, had been largely spared in Syria's 14 months of bloody uprising. But recent protests and violence there could signal a significant shift.
Some analysts said recent attacks raise concerns about the presence of jihadist elements in Syria, noting Thursday's Damascus strikes resemble suicide car bombings during the sectarian violence in Iraq in the past decade.
But opposition groups have said the regime is responsible for the violence that erupted after government forces began a crackdown on peaceful protests in March 2011. That fierce clampdown spurred a grassroots uprising against the regime.
The opposition Syrian National Council said al-Assad's regime staged Thursday's deadly suicide bombings in Damascus "to spur chaos, disrupt the work of the international observers and divert attention away from other crimes being committed by its forces elsewhere."
"In orchestrating such acts," the council said Friday, "the regime seeks to prove its claims of the existence of 'armed terrorist gangs' in the country that are hindering its so-called 'efforts of political reform.'"
The deadly blasts in Damascus took place near a military intelligence center.
But the SNC questioned how the attackers could have made it past security to conduct the bombings.
More than 1,000 people have died since a cease-fire was supposed to go into effect on April 12, according to the LCC.
CNN cannot independently verify reports of deaths and violence because the Syrian government has severely restricted access to international media.
A team of U.N. monitors is on the ground to observe the progress of the cease-fire and encourage the implementation of a U.N.-backed peace plan.
On Saturday, the head of the U.N. observer mission, Maj. Gen. Robert Mood, said 157 civilian and military monitors were in the provinces of Daraa, Idleb, Hama and Homs, the state-run Syrian Arab News Agency reported. About 300 observers are expected within weeks.
more armored Jeeps and another 13 more, bringing to 25 the number of vehicles to arrive on Saturday, all offered by the European Union.
CNN's Kamal Ghattas, Saad Abedine and Salma Abdelaziz contributed to this report.
