International Atomic Energy Agency (IAEA) director general Mohamed ElBaradei is visiting India.
Indian nuclear plants are to be opened to inspection
He is due to meet senior government leaders including foreign minister Pranab Mukherjee in Delhi on Wednesday.
The visit comes as a political row about India's controversial nuclear deal with the US is threatening to bring down the government.
Communists and others say the deal will undermine India's independent foreign policy and nuclear weapons programme.
The BBC's Chris Morris in Delhi says the Indian government is keen to emphasise that Mr ElBaradei is on a scheduled goodwill visit, not connected to the nuclear deal.
The row over the deal could bring early elections
Nevertheless, the government's communist allies are keeping a close eye on what he is doing, our correspondent says.
Soon, the government will have to finalise negotiations with the IAEA on an agreement to put civilian nuclear reactors under UN safeguards.
That has to be done before the deal with the United States goes ahead.
For the communists, however, it is unacceptable. If no compromise can be found, the government could fall, prompting early elections.
The nuclear deal gives India access to civilian nuclear fuel and technology from abroad, even though it has not signed the nuclear Non-Proliferation Treaty.
In the United States, some members of Congress argue that India is, in effect, being rewarded for ignoring the rules.
The president of India's governing Congress party, Sonia Gandhi, says India needs the agreement to help meet the country's huge energy demands.
She has described its critics as enemies of development.