AMMAN/BEIRUT China accused Western countries of stirring civil war in Syria and two Iranian warships docked at a Syrian naval base, underscoring rising international tensions over the near year-long crisis.
Despite pursuing a sustained military crackdown on the opposition in cities across the country, President Bashar al-Assad forged ahead with plans to hold a referendum at the end of the week.
Activists in the western city of Hama said troops, police and militias had set up dozens of roadblocks, isolating neighbourhoods from each other.
"Hama is cut off from the outside world. There are no landlines, no mobile phone network and no internet. House to house arrests take place daily and sometimes repeatedly in the same neighbourhoods," an opposition statement said.
Government troops extended their control on Hama after an offensive last week that concentrated on northern neighbourhoods on the edge of farmland that have provided shelter for Free Syrian Army rebels.
The rebel fighters have been attacking militiamen, known as shabbiha, while avoiding open confrontations with armoured forces that had amassed around Hama.
Government forces also maintained their siege of pro-opposition neighbourhoods of Homs, south of Hama on the Damascus-Aleppo highway. Opposition activists reported sporadic morning shelling of Baba Amro district.
Security forces also mounted a campaign of arrests and raids in two suburbs of Deraa city and loud gunfire was heard, activists said. The reports could not be independently verified.
In Damascus, activists unfurled a pre-Assad era national flag over a road bridge at the edge of the capital, YouTube footage showed.
The Monday actions followed a weekend which saw one of the biggest demonstrations yet in Damascus as the uprising against Assad's 11 year-rule neared its first anniversary.
Security forces have killed at least 5,000 people, according to human rights groups, in a campaign to crush the revolt while the Assad government says it has lost more than 2,000 soldiers and security agents in what it describes as a struggle against foreign-backed terrorists,
The conflict has also pitted Western and Gulf-led Arab powers against Assad allies Russia, China and Iran.
The former have condemned Assad for the bloodshed and called for him to step down. Beijing and Moscow say all sides are to blame for the violence and the crisis should be resolved through talks, not foreign intervention.
China's Communist Party mouthpiece the People's Daily, in a front page commentary on Monday, said: "If Western countries continue to fully support Syria's opposition, then in the end a large-scale civil war will erupt and there will be no way to thus avoid the possibility of foreign armed intervention."
A Chinese envoy met Assad in Damascus on Saturday and backed his plan to hold a referendum this coming Sunday on a new constitution which would lead to multi-party parliamentary elections within 90 days.
Syria's official SANA news agency said about 14,600,000 people throughout the country were eligible to take part in the referendum. The West and Syrian opposition figures have dismissed the plan as joke, saying it is impossible to have a valid election amid the continuing repression.
Assad has ruled Syria for 11 years after succeeding his father Hafez on his death. The Assad family belongs to the Alawite sect, an offshoot of Shi'ite Islam, in a majority Sunni country, and there are fears the uprising could break down into a full sectarian conflict.
Meanwhile two Iranian naval ships docked at the Syrian port of Tartous on Saturday, Iran's state-run Press TV reported. The ships were said to be providing training for Syrian naval forces under an agreement signed a year ago.
Iranian Defence Minister Ahmad Vahidi, quoted by the semi-official Fars news agency, said: "Our ships passed through the Suez canal and it is Iran's right to have a presence in international waters."
With Shi'te-led Iran already at odds with the United States, Europe and Israel over its nuclear programme, the deployment was likely to add to Western concerns that the Syria crisis could boil over into a regional conflict if it not resolved soon.
Israeli Deputy Prime Minister Dan Meridor, asked about the Iranian move at briefing in Jerusalem, said Assad was supported by Iran and the Hizbollah militant group and that Russia and China had given him "a licence to kill".
"Iran invests a lot in trying to save the Syrian regime, by advising them, by supplying equipment, by sending people and maybe by also sending these ships through the Suez Canal," he said.
Foreign ministers at a G20 industrialised and emerging nations meeting in Los Cabos, Mexico, said they were worried about whether a peaceful solution could be found.
The West has ruled out any Libya-style military intervention but the Arab League, led by Saudi Arabia, has indicated some of its member states were prepared to arm the opposition.
In Washington the senior U.S. military officer, General Martin Dempsey, said intervening in Syria would be "very difficult" because it was not like Libya.
Syria's army is very capable, with a sophisticated, integrated air defence system and chemical and biological weapons, Dempsey said. It was also not clear who or what the fragmented opposition was exactly, he said.
A so-called "Friends of Syria" conference is scheduled to take place in Tunisia this Friday, bringing together Western and Arab powers.
Australian Foreign Minister Kevin Rudd, speaking in Los Cabos, said the group aims "to place maximum pressure on president Assad to go, to end the butchery that we see day by day unfolding in Syria and to make sure we have a durable and peaceful political transition".
(Additional reporting by Khaled Yacoub Oweis in Amman, Parisa Hafezi in Tehran; Susan Cornwell in Washington; Krista Hughes in Los Cabos, Mexico; Editing by Giles Elgood)