MOSCOW — International diplomatic pressure increased on Moscow on Friday to end the joint Russian-Syrian siege of the city of Aleppo, but Moscow’s U.N. ambassador says he will most likely veto a U.N. Security Council resolution that would ground Russian warplanes.

Russia’s parliament meanwhile ratified a treaty with Syria that allows its troops to stay indefinitely in the country, a show of support for embattled Syrian President Bashar Assad.

The siege by Syrian forces backed by Russian warplanes has inflicted immense suffering on civilians in the city’s rebel-held eastern districts. A cease-fire brokered by the United States and Russia collapsed last month and Washington-Moscow ties have deteriorated sharply; Russian lawmakers said ratifying the treaty with Syria on Friday was a necessary step to stand up to the U.S.

The United States and Russia support opposite sides in the more than 5-year-old war – Moscow has been a staunch Assad ally and Washington backs Syrian rebels trying to oust him.

As Aleppo’s misery dragged on, Russia’s United Nations ambassador Vitaly Churkin rejected a French-proposed U.N. Security Council resolution that would call for grounding all aircraft, including Russia’s, over Aleppo.

The resolution, also calling for an Aleppo cease-fire, is to be considered Saturday but Churkin said, “I cannot possibly see how we can let this resolution to pass.”

In a last-minute move Friday, Russia introduced its own draft resolution urging “immediate, safe and unhindered humanitarian access throughout Syria,” similar language to the French text.