Russia's United Aircraft Corporation (UAC) is to introduce a fourth Sukhoi T-50 stealth fighter into its test and development program this year, the company's President Mikhail Pogosyan said on Thursday.

"We are now testing three aircraft. A fourth will be brought in this year," he said.

Pogosyan had said earlier this year that the firm would introduce a fourth aircraft into the test program but did not disclose when.

The first production standard T-50 is due to enter service with the Russian Air Force by 2015, and the first evaluation example by 2013. The service plans to acquire 60 of the fifth-generation fighters.

The T-50, also known as project PAK-FA, first flew on January 29, 2011 and was first publicly revealed at the Moscow Air Show in August that year. India will also acquire an advanced fighter aircraft based on the T-50.