US President Donald Trump has turned down India's invitation to be the chief guest at next year's Republic Day parade.

Sources said the US authorities conveyed Trump's decision in a letter to NSA Ajit Doval recently. The US president's "domestic commitments", including his State of the Union address, around that time seems to have been the reason for declining the invitation. In the letter, Trump is said to have regretted his inability to accept the invitation.

India had officially sent the Republic Day invite to the US president in April this year. The US authorities had confirmed receiving the invite, but said the final decision would be taken after the inaugural 2+2 dialogue in September.

Ties between India and the United States have recently been under a lot of strain because of India's defence purchases from Russia and the oil imports from Iran.

US had said that countries trading with Russia's defence and intelligence sectors would face automatic sanctions under a sweeping legislation called Countering America's Adversaries Through Sanctions Act (CAATSA). However, despite the threat, India went ahead and bought the formidable S-400 surface to air missile systems from Russia.

The Trump administration had also given a November 4 deadline to its allies to bring down their import of Iranian oil to zero, but New Delhi continued to import the oil. State Department spokesperson Heather Nauert said had said that US was reviewing India's decision "very carefully".

According to a Times of India report, at one stage, India was so keen to host Trump that it offered to consider another set of dates for the visit in the month of February to avoid clash with the State of the Union.