US President Donald Trump - who has in the past praised India as a "great country" and its leader, Prime Minister Narendra Modi, as a "true friend" - Thursday night described India and its air as "filthy" during the last presidential debate with his Democratic challenger Joe Biden while discussing the environment and the Paris climate change agreeement.

"Look at India, it's filthy, it's air is filthy," Trump said on Thursday night in Nashville, Tennessee, while comparing it to China and Russia to what he said was the condition of the US environment.

"We have the best carbon emission numbers that we've had in 35 years under this administration, we are working so well with industry," he said before lunging at India and the other two countries.

That was the only mention of India during a debate that mostly stayed clear of foreign policy and strategic interests.