"Given the security situation in Colombo, it is important for India to address all possibilities because the Prime Minister (Manmohan Singh) will be attending," a top diplomatic source said.
He said war ships with helicopters will be anchored off Sri Lanka's coast in case the island's only international airport was forced to shut.
The airport has been attacked several times by the rebel Liberation Tigers of Tamil Eelam (LTTE).
Sri Lanka has closed its air space and diverted regular commercial flights to neighbouring India and the Maldives when the Tigers flew their light aircraft on bombing missions in Colombo and elsewhere.
The Tigers also operate a fleet of speed boats and have staged attacks against Sri Lankan ports.
Diplomats said security arrangements for the eight-member South Asian Association for Regional Cooperation (SAARC) summit were discussed nine days ago between Colombo and New Delhi and an agreement reached.
There was no formal word from the Sri Lankan authorities about the Indian security cover for the summit, which will gather leaders from Afghanistan, Bangladesh, Bhutan, India, the Maldives, Nepal, Pakistan and Sri Lanka.
Many roads in the capital are closed and access limited to authorised vehicles amid fears of attacks by the Tamil Tigers, who are fighting for an independent homeland for the island's Tamil minority concentrated in the northeast.
Security forces are locked in combat with the Tigers in the north. A daily death toll is reported from the area and fighting has escalated since Colombo pulled out of a Norwegian-arranged truce in January.
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