"The government has taken note of the 'sub-critical' underground nuclear test conducted by the USA on July 2, 1997 and is concerned that this has been justified 'as an activity permitted under the CTBT'," the Foreign Ministry said in a statement.
"It is a matter of regret that, as it has now emerged, the CTBT contains loopholes which are exploited by some countries to continue their testing activity, using more sophisticated and advanced techniques," the ministry said.
India, the first country to propose a global ban on all nuclear testing in 1954, last year refused to endorse the CTBT on the grounds that it permitted nuclear powers to refine their arsenals and did not commit them to disarm.
"Recent developments confirm the validity of India's concerns expressed during the CTBT negotiations as well as our eventual decision last year that India could not be party to such a treaty," the Foreign Ministry said.
New Delhi says the pact is flawed because it allows the five nuclear powers-- Britain, China, France, Russia and the United States -- to design nuclear arms through laboratory testing. It says the treaty is a discriminatory non-proliferation measure that does not work towards global nuclear disarmament.
India, wedged between nuclear-armed China and nuclear-capable Pakistan, conducted its only nuclear test in 1974 after three wars with Pakistan and one with China.
"India further reiterates that mere non-proliferation arrangements as are now manifest do not take into account our legitimate security concerns," the Foreign Ministry said.
New Delhi says it has no nuclear weapons but experts say both India and Pakistan can swiftly assemble nuclear arms.
The United States says its latest experiments do not trigger the kind of nuclear chain reaction banned by the CTBT.
It said the tests, which involve massive supercomputer calculations, were needed to assess how age would affect the materials in the U.S. nuclear arsenal and predict weapons performance without actual explosions.
"What we've come up with is a nuclear test explosion ban treaty," a senior Indian Foreign Ministry official said.
"If they wanted only to ban explosions, they should have taken it to a conference on environment, not the (United Nations) Conference on Disarmament," he added.