Thousands of protesters in war-torn Syria's opposition enclave of Idlib on Tuesday marked 11 years since the start of an anti-regime uprising, buoyed by the global outcry over Russia's invasion of Ukraine.

Gathered on the main square in the northwestern city of Idlib, more than 5,000 people took part in one of the largest rallies the beleaguered region has seen in months.

Many of the demonstrators hoped the war launched by the Syrian regime's main backer Russia in Ukraine would rekindle interest in their cause.

"What is happening in Ukraine today is similar to the situation here; the enemy is the same and the goal is the same," protester Radwan Atrash told Agence France-Presse (AFP).

Thousands of demonstrators marked the date in other cities across opposition-held northern Syria.

Bashar Assad's grip on power was held by a thread after a nationwide uprising that erupted on March 15, 2011, but escalated into a full-fledged civil war.

But a decision by his Russian counterpart Vladimir Putin to throw his military might behind the Syrian regime changed the course of the conflict and saved Assad's hold on power.

The war left half a million people dead, mostly in attacks by the regime and its allies, including both Russian and Iranian forces, as well as a myriad of militia groups. On the other hand, Turkey has been the main backer of the opposition in the region and has been leading the humanitarian efforts for those who had to flee from the regime attacks.

Around 4 million people, at least half of them displaced, now live in a region of northwestern Syria that is the last enclave fighting Assad's rule despite years of deadly Russian-backed offensives.

A few Ukrainian flags were visible at the Idlib protest, as were banners expressing solidarity with the Ukrainian people and demanding action against Putin.

A medic among the protesters at the city's main roundabout had some advice for his counterparts in Ukraine.

"Fortify your hospitals with cement blocks; the enemy Putin does not distinguish between civilians, wounded people and fighters," said Ali Hamoush, who works at an Idlib hospital.

Russia has repeatedly targeted medical facilities in Syria, according to witnesses, medics and human rights groups.

A pediatric hospital was hit by an apparent Russian strike in the besieged Ukrainian city of Mariupol last week, fuelling accusations of war crimes against Putin.

As the conflict drags on, rights groups have pleaded for the international community not to forget Syria.

"While we look with shock and horror at what is unfolding in Ukraine, we are reminded of the intense and worsening suffering that the Syrian population has endured," Jan Egeland, secretary-general of the Norwegian Refugee Council, said this week.

"One of the greatest human tragedies of our time has gotten worse over the last year in the shadow of crises elsewhere."

Syria's economy has been battered by a decade of conflict and grueling sanctions.

"The coincidence of this year's anniversary with the appalling Russian aggression against Ukraine ... highlights Russia's brutal and destructive behavior in both conflicts," Britain, France, Germany, Italy and the United States said in a joint statement.

Last week, the U.N. commission for inquiry on Syria called for "a review of the implementation and impacts of sanctions currently imposed on Syria" in light of deteriorating living conditions.

But the five nations said they do "not support efforts to normalize relations with the Assad regime."

Assad is among the few heads of state to openly support Putin's invasion of Ukraine.

Moscow is currently recruiting thousands of fighters in Syria, from the regular army and from militia groups, to be put on standby for possible deployment in Ukraine.

The stiff resistance faced by invading Russian troops and Putin's growing pariah status appeared to galvanize the Idlib crowd.

"My message to the Ukrainian people is 'Don't give up.' Eleven years have passed, but we are undaunted and, God willing, victory is ours," said protester Salwa Abdelrahman.