President Joe Biden has been encouraged by two US senators to lift sanctions against India over its purchase of Russia’s S-400 air defense system, claiming that such a measure would jeopardize growing cooperation.

In 2018, India and Russia agreed on a $5.5 billion deal for five surface-to-air missile systems to defend against long-time adversaries Pakistan and China, with whom it is involved in a border dispute.

However, the proposed transfer has generated a rift with the United States, which passed a bill in 2017 imposing sanctions on any country that cooperates with Russia’s defense and intelligence industries.

Senators John Corny and Mark Warner, both Republicans, wrote to Biden on Tuesday, requesting a waiver based on national security and increased collaboration.

“We believe there is a national security imperative to waiving sanctions,” the senators said in their letter which they issued in a press release.

They said they were concerned the transfer of the Russian systems would trigger sanctions against India under the Countering America’s Adversaries Through Sanctions Act (CAATSA), which was enacted to hold Russia accountable for interfering in U.S. elections, cyber hacking and bullying Ukraine.

India has made a down payment on the S400 systems and the first set of missile batteries is expected to be begin deployment later this year.

The United States imposed sanctions on Turkey for buying the same equipment last year.

Cornyn and Warner, who are co-chairs of a Senate India Caucus, said they shared the administration’s concerns about Russia but they warned of damage to cooperation with India if sanctions were to be imposed.

“We believe that the application of CAATSA sanctions could have a deleterious effect on a strategic partnership with India, while at the same time, not achieve the intended purpose of deterring Russian arms sales,” they wrote.

India has been cutting back on purchases of military equipment from Russia, which for years was its main source, with a 53% drop in Russian arms exports to India from 2016 to 2020 compared with the preceding five-year period.

India’s defence deals with the United States, on the other hand, have been increasing with sales at $3.4 billion in 2020 financial year. These are positive trends, the senators said.

“Imposing sanctions at this time could derail deepening cooperation with India across all aspects of our bilateral relationship – from vaccines to defense cooperation, from energy strategy to technology sharing.”