India on Monday warned it would oppose the UN treaty banning nuclear tests, calling it disciminatory, despite President Barack Obama's hopes that the United States will ratify it.

Shyam Saran, India's special envoy on nuclear issues, conceded on a visit to Washington that the Comprehensive Nuclear Test Ban Treaty (CTBT) could prove to be a "contentious" issue with the new US administration.

The CTBT would ban all nuclear explosions for any purpose. It cannot come into effect as nuclear powers such as the United States and China have not ratified it or, in the case of India and rival Pakistan, even signed it.

Saran said India opposed the CTBT because it "was not explicitly linked to the goal of nuclear disarmament."

"For India, this was crucial since it was not acceptable to legitimize, in any way, a permanent division between nuclear weapons states and non-nuclear weapons states," he said at the Brookings Institution.

India declared itself a nuclear state in 1998, arguing it was unfair that international treaties only allowed five nations -- Britain, China, France, Russia and the United States -- to maintain the ultra-destructive weapons.

Former US president George W. Bush nonetheless reached a deal giving India access to civilian nuclear technology, triggering protests by some members of Obama's Democratic Party.

Obama in his campaign platform said he would encourage the US Senate to ratify the CTBT and encourage other nations to do so. The United States has not conducted a nuclear test since 1992.

Obama said he would work for the eventual abolition of nuclear weapons, heartening anti-nuclear activists including survivors of the world's only atomic attacks in the Japanese cities of Hiroshima and Nagasaki in 1945.

Saran said that if "the world moves categorically towards nuclear disarmament in a credible time-frame, the Indo-US differences over the CTBT would probably recede into the background."