In the year since Russia began conducting airstrikes in support of the Syrian government, the intervention has worked to secure two of the most important Russian objectives.

No longer is President Bashar al-Assad’s hold on power in ­Damascus seriously challenged by the five-year-old rebellion against his rule. Russia’s role as a regional and global power, a vital player in any effort to resolve the Syrian war, has also been assured.

A third key goal remains elusive. The military support to ­Assad has not yet proved sufficient either to defeat the anti-Assad rebellion or to force a settlement on Russian terms.

But there is no suggestion yet that Russia is finding itself in the “quagmire” President Obama predicted would ensue when the intervention first got underway a year ago, nor is Moscow showing any sign of tiring of the military engagement.

Rather, Russia now appears to be doubling down on its military support for Assad, with the violence escalating and no end in sight.

Negotiations with the United States for a cease-fire have collapsed amid angry recriminations, casting into doubt whether cooperation between Moscow and Washington on Syria can work to provide a way out of the conflict.

Russian warplanes are backing a fierce assault to recapture eastern Aleppo, the bloody epicenter of the struggle for control of Syria. The bombs rained down with renewed ferocity on Friday, including banned phosphorus and cluster bombs that struck residential neighborhoods and two more hospitals, according to medical workers and residents of the city.

The World Health Organization said Friday that 338 people have been killed in the airstrikes that began last week, more than 100 of them children.

Altogether, more than 3,000 civilians have been killed in Russian airstrikes in the year since they were launched, human rights monitors said Friday. They gave slightly differing numbers, with the Britain-based Syrian ­Observatory for Human Rights putting the toll at 3,804 and the Syrian Network for Human Rights at 3,624.

It seems certain to rise. On Friday, the Kremlin indicated that Russia has no intention of winding down its involvement in Syria, saying that President Vladimir Putin has put no time frame on an intervention Moscow had initially indicated would last three months.

“If you remember exactly, Russian Supreme Commander-in-Chief and President Putin never voiced any tentative time frameworks after he made the decision to begin this operation of the Russian Aerospace Forces,” Kremlin spokesman Dmitry Peskov told reporters in Moscow, when asked whether Russia had expected the military engagement to last so long.

The intervention has worked, Peskov said, because the terrorist groups the Islamic State and the al-Qaeda affiliate formerly known as Jabhat al-Nusra have not succeeded in capturing the capital, Damascus.

“One thing can be stated: Considering that ISIL, al-Qaeda and Nusra are not seated in Damascus, this is probably the main positive result of the support our aviation has been giving to the legitimate Syrian Armed Forces.”

Progress against the anti-Assad rebellion has been slower elsewhere in the country. Battles to reclaim territory in the northwestern province of Latakia went well earlier in the year but have stalled short of the Turkish border. The effort to impose a complete siege on the rebel-held portion of Aleppo, an objective of the Syrian government ever since rebel fighters seized control of the eastern half of the city in 2012, has dragged on for most of the year. It came at the cost of hundreds of lives among Syrian army troops and the Iranian-backed Shiite militias from Iraq and Lebanon that are providing much of the manpower for the offensive.

As the noose on Aleppo has tightened, the rebels have been making significant gains in the province of Hama, seizing a string of villages and moving within six miles of the city of Hama, once the biggest center of the peaceful protests against ­Assad until they were quelled five years ago.

That calls into question whether Syria’s government will ever be able to fully vanquish the rebellion, even with Russian support. The Syrian effort “is like putting a handkerchief on a plate,” said Salman Shaikh, a consultant who is participating in second-tier negotiations between regime and rebel supporters. “You can move the handkerchief, but you will never cover the plate.”

It may, however, be an objective Russia supports, said Jeff White of the Washington Institute for Near East Policy. “An outright military victory is a long way off, but incrementally they seem to be aiming at that,” he said. “The position of the military opposition hasn’t improved at all since the spring of 2015, and the regime hasn’t suffered any drastic setbacks in that time.”

A U.S. policy predicated on the assumption that Russia would realize it can’t win the war and would therefore negotiate a settlement is meanwhile seriously in doubt.

If the Obama administration believes Russia will become stuck in Syria and will want to get out, that moment does not appear to have arrived, said Robert Ford, the former U.S. ambassador to Syria who is now at the ­Washington-based Middle East Institute.

The Russian footprint in Syria remains relatively light, and though it includes advisers on the ground, is mostly confined to airstrikes. It is costing the Kremlin an estimated $3 million a day, a small fraction of its $55 billion annual defense budget. Though Syria might well be a quagmire, Ford said, “it is a sustainable quagmire.”

Russia’s intention may be at least to take Aleppo before engaging in serious negotiations with the United States, Shaikh said. Losing Aleppo would be a severe blow to the rebels, depriving them of a foothold in any major Syrian city, he said. Russia would then be in a position to negotiate a settlement more favorable to Assad with the United States and its allies in the opposition — perhaps after a new administration takes office in Washington.

Military preparations for an all-out assault on the city appear to be intensifying. Iraqi Shiite militia reinforcements have been converging on the outskirts of Aleppo to bolster the front lines. Akram al-Kaabi, the leader of the biggest of the militias, Harakat al-Nujaba, visited them in person this week. A Russian aircraft carrier, the Admiral Kuznetsov, will arrive in the Mediterranean this month, which will give the Russian air force additional capacity to wage airstrikes.

Russia has already been deploying new and heavier weaponry in the assault on Aleppo, including the phosphorus, cluster and bunker buster bombs that have deepened the misery in the city over the past week, according to eyewitness accounts and allegations by senior U.S. and U.N. officials.

Russian Foreign Minister Sergei Lavrov denied on Friday that Russia has deployed such weapons, whose use U.N. Secretary General Ban Ki-moon said earlier this week would constitute a war crime.

But photographs posted on social media by activists in eastern Aleppo suggested otherwise. They showed clusters of glowing fires illuminating the deserted streets, ignited by the bombs’ incendiary components. Several blazed outside the Al Zahra hospital, eastern Aleppo’s last remaining maternity hospital, which was among the locations hit on Friday.

Zakaria Zakaria in Istanbul contributed to this report.