Hong Kong CNN  — 

When Chinese leader Xi Jinping and Russian President Vladimir Putin sit down with other Asian leaders at a summit in Central Asia Friday, they will seek to present a united front to counterbalance the United States and its allies.

But within their “no-limits” relationship, potential differences over Russia’s faltering invasion of Ukraine are beginning to emerge. The brutal war has also created friction within the Shanghai Cooperation Organization (SCO), a regional security grouping led by Beijing and Moscow.

Beginning Thursday, leaders from the grouping, including India, Pakistan, Iran and four Central Asian countries, have convened in the city of Samarkand in Uzbekistan, which has seen a flurry of high-level talks, including the first face-to-face meeting between Xi and Putin since the invasion.

During their talks, Putin conceded that Chinese officials had “questions and concerns” over his protracted military assault, in what appeared to be the first – although somewhat veiled – admission of their differences on the conflict.

Putin’s appearance alongside Xi at the high-profile summit comes just days after Russian forces suffered a series of major defeats on the Ukrainian battlefield. Moscow’s invasion has left it diplomatically isolated, and its economy severely weakened by a raft of punishing Western sanctions.

In recent months, China has offered Russia tacit support and stepped up economic assistance to its neighbor, boosting bilateral trade to a record high. But as the conflict drags on into winter, analysts question how far Xi will be prepared to go in continuing to back Putin – and at what cost.