NEW DELHI — Since Russian President Vladimir Putin unleashed a large-scale attack against Ukraine on Thursday, the Biden administration has sought to rally allies and partners to condemn Russian aggression and join in trade and financial sanctions.

One country that has conspicuously rebuffed Biden’s appeals is India, a rising Asian power that relies on Moscow for almost all its advanced weapons.

For years, India has juggled its close relations with Russia — an enduring legacy of the Cold War — with its fast-growing ties with the United States, which has envisioned India as a crucial partner in its long-term strategy to counter China’s rise.

But India’s balancing act is proving increasingly difficult this week as Russian tanks and fighters bear down on Kyiv in a war that has drawn a thick line between the West and Russia, with only China as Moscow’s major economic and diplomatic backer.

While Japan, Australia and the United States all unveiled new export bans against Russia on Thursday and Friday, India — the fourth leg of the grouping known as the Quad — demurred, highlighting a glaring fissure in one of the key American partnerships that Biden has pledged to repair and strengthen.

Facing pressure from both sides, India on Friday joined China and the United Arab Emirates in abstaining from a United Nations Security Council resolution that would have condemned the invasion. Russia vetoed the measure.

In remarks Thursday, Biden urged countries to take a stand against Putin, saying that “any nation that countenances Russia’s naked aggression against Ukraine will be stained by association.” The United States was pressing India about its stance toward Russia, Biden told reporters. “We haven’t resolved that completely,” he added.

Shortly after Biden spoke, the State Department said Secretary Antony Blinken held a call with his Indian counterpart, Subrahmanyam Jaishankar, to discuss “the importance of a strong collective response to Russian aggression.” India issued only a terse acknowledgment that the call took place.

Meanwhile on Thursday, Indian Prime Minister Narendra Modi spoke with Putin and called for “concerted efforts from all sides to return to the path of diplomatic negotiations,” according to a readout from the Indian government. Modi’s language diverged sharply from the Western characterization of the Russian attack as a one-sided, unprovoked invasion of a sovereign nation.

Derek Grossman, a senior defence analyst at the Rand Corp. think tank, said the Quad grouping — a cornerstone of Biden’s Indo-Pacific strategy to counter China — “could easily fray.” In recent weeks, other Quad nations have condemned Moscow as Russian troops massed around Ukraine — but not India.

The Russian operation “is clearly breaking the rules-based order, which is whole reason the Quad got together in the first place,” Grossman said. “For India to continue to sit on the sidelines — that’s going to become increasingly an untenable position if they want to maintain good ties not just with the Quad but also Europe.”

Indian sympathies for Russia — and Russia’s support for India — reach back to the early decades of the Cold War, when Washington often sided with India’s archrival, Pakistan, over issues including the contested Kashmir region. In 1971, when India fought Pakistan over Bangladesh, the United States backed Pakistan in part because it was a crucial party to a plan by President Richard Nixon to secretly establish relations with China. In the ensuing years, Indian military imports from the Soviet Union soared, while at the diplomatic level, it maintained a stated policy of nonalignment.

Today, Russia has leased a nuclear submarine to India. Russian scientists are helping develop India’s hypersonic missile program. Russian T-90 tanks form the backbone of India’s ground forces, and Russian MiG and Sukhoi fighter jets are mainstays in its air force. The Indian navy’s flagship is an aircraft carrier — a “Kiev-class” — purchased from Russia.

According to the Stockholm International Peace Research Institute, Russia is by far India’s largest arms supplier, accounting for 70 percent of India’s imports between 2011 and 2015 and roughly half between 2015 and 2020. India recently purchased Russian S-400 antiaircraft missiles, which could trigger U.S. sanctions.

As fighting erupted Thursday in Ukraine, Russian and Ukrainian diplomats in New Delhi presented duelling pitches. The ranking Russian envoy, Roman Babushkin, praised Modi’s “independent and balanced” approach and offered a reminder of what is at stake. “Russia is the only country which is sharing sophisticated technologies with India and defence cooperation between us is a strong factor for international peace and stability,” he told Indian media outlets. “We have big plans and we hope that our partnership will continue at the same level which we are enjoying today.”

The Ukrainian ambassador to India, Igor Polikha, told reporters that he is “deeply dissatisfied” with India’s position, which he attributed to its “special, privileged, strategic relation with Russia.” He pleaded for Modi’s help to restrain Putin at what he called “the moment of destiny.”

“I don’t know how many world leaders Putin may listen to, but the stature of Modi-ji makes me hopeful,” Polikha said, using an honorific for the Indian leader. “We are waiting, asking, pleading for the assistance of India.”

Meanwhile, India’s foreign policy circles remained deeply ambivalent about the course ahead. Even though maintaining neutrality angered Washington, it would be equally difficult for India to alienate Moscow now, said Sushant Singh, a fellow at the Centre for Policy Research, a think tank in New Delhi.

“No one else is going to give you a nuclear submarine,” he said. “Who else is going to sell India an aircraft carrier?”

And yet, one other realpolitik consideration could tip India’s hand, Singh added: India now considers China — which is increasingly embracing Russia diplomatically and purchasing more Russian energy and now wheat — to be its biggest threat and one that could be countered only with American help.

“India has never been caught in this kind of an either-or situation,” Singh said. “It’s the biggest diplomatic challenge since the Cold War.”

Others were more dismissive of the prospect of India joining the Western bloc.

“We owe answers to no one except ourselves,” D.B. Venkatesh Varma, who served as India’s ambassador to Moscow until last year, wrote in a column in the Indian Express. “Russia has not covered itself in glory. But that is no reason to doubt the merits of our long-standing relations with it — just as we held our noses and deepened our relations with the U.S. during its decade-long intervention in Iraq.”