Adm. Phil Davidson speaks at a change of command ceremony in which Rear Adm. Jeff Hughes relieved Rear Adm. Roy Kitchener, commander, Expeditionary Strike Group 2, aboard the amphibious dock landing ship USS Gunston Hall (LSD 44), July 21, 2017. (U.S. Navy photo/Dana Legg)

Adm. Phil Davidson speaks at a change of command ceremony in which Rear Adm. Jeff Hughes relieved Rear Adm. Roy Kitchener, commander, Expeditionary Strike Group 2, aboard the amphibious dock landing ship USS Gunston Hall (LSD 44), July 21, 2017. (U.S. Navy photo/Dana Legg)

Adm. Phil Davidson, the nominee to take over U.S. Pacific Command, has warned against sanctioning India for buying Russian arms that could include a proposed $6 billion deal for S-400 Triumf anti-air missiles billed as "F-35 killers" by Moscow.

The growing strategic partnership between the U.S. and India to counter China should take precedence over sanctions, Davidson said in written responses to questions ahead of his Senate Armed Services Committee (SASC) confirmation hearing Tuesday.

Davidson noted his "concern for our defense relationships in the Indo-Pacific with countries such as India, Vietnam, and Indonesia."

"If the United States decides to sanction these partner nations for their purchases of Russian equipment, this decision may hinder the growth of each developing partnership and increase each partner's dependence on Russia," said Davidson, currently commander of Fleet Forces Command.

The immediate issue was the proposed deal, now in its final stages, for the S-400s that Russian President Vladimir Putin first pitched to Indian Prime Minister Narendra Modi on a visit to India in 2016.

Last week, Indian Defense Minister Nirmala Sitharaman was in Moscow to work out details on the reported $6 billion purchase of the S-400s, a mobile surface-to-air defense system dubbed the SA-21 "Growler" by the U.S. and NATO.

In what could be seen as a sales pitch, Russian officials earlier this week claimed that the U.S., French and British cruise missile strikes on suspected Syrian chemical weapons sites avoided areas protected by S-400 and S-300 systems.

India's interest in the S-400s has intensified with reports that Russia has already begun deliveries of the S-400 systems to regional rival China under a $3 billion deal for the missiles that Moscow claims would be effective against stealthy U.S. F-35 Joint Strike Fighters.

The proposed S-400 deal could potentially make India liable under the Countering America's Adversaries Through Sanctions Act (CAATSA) signed by President Donald Trump last August. The Act targeted Russia for its interference in the 2016 presidential elections and its aggression in Crimea and Ukraine.

The Act required the president to sanction any country that strikes "highly-significant" agreements with Russia's defence industry.

In addition, the State Department last October listed more than three dozen Russian companies and warned businesses and nations worldwide that those dealing with the Russian firms could face U.S. sanctions. The list of Russian firms included missile manufacturer Almaz-Antey, maker of the S-400.

In his written responses to SASC, Davidson urged Congress to consider the long history of India and other regional nations in buying equipment and weapons from the former Soviet Union and now the Russian Federation before moving to consider sanctions under CAATSA.

He added that "Russian operations and engagements throughout the Indo-Pacific continue to rise -- both to advance their own strategic interests and to undermine U.S. interests. Russia also sees economic opportunities to build markets for energy exports and arms sales in the region."

But CAATSA sanctions were not the answer, he said.

The U.S. also faces a dilemma on whether to impose sanctions on NATO ally Turkey for its own estimated $2.5 billion deal to purchase S-400s from Russia. Putin was in Ankara earlier this month to seal the deal with Turkish President Recep Tayyip Erdogan.

Several other nations, including Iraq, have expressed interest in acquiring the S-400s rather than the U.S. Patriot surface-to-air missile defense system made by Raytheon.

The interest in the S-400s has been heightened by the perceived failure of the U.S. Patriot systems to protect the Saudi Arabian capital of Riyadh against missile launches by Houthi rebels in Yemen.