Sri Lankan officials said on Tuesday security forces were maintaining a high level of alert after intelligence reports that militants were planning fresh attacks before the start of the holy month of Ramazan.

The head of the police ministerial security division had said in a letter to lawmakers and other officials that attacks were expected on Sunday or Monday by militants dressed in army uniform.

There were no attacks on Sunday and Monday but security across Buddhist-majority Sri Lanka remains ramped up, with scores of suspected militants arrested since the April 21 attacks on hotels and churches that killed more than 250 people, including 42 foreign nationals.

"Security will stay tight for several days because military and police are still tracking down suspects," a senior police intelligence official said on Tuesday.

Another government source told Reuters a document has been circulated among key security establishments instructing all police and security forces across the Indian Ocean island nation to remain on high alert because the militants were expected to try a strike before Ramazan.

Ramazan is scheduled to begin in Sri Lanka on May 6.

The government has also banned women from wearing face veils under an emergency law that was put in place after the Easter Sunday attacks.

Authorities suspect members of two previously little-known groups — National Thawheedh Jamaath (NTJ) and Jammiyathul Millathu Ibrahim — of carrying out the attacks, although the Islamic State extremist group has claimed responsibility.

Authorities believe Zahran Hashim, the founder of NTJ, was the mastermind and one of the nine suicide bombers.

Read: Suspected mastermind of Sri Lanka attacks spent 'substantial time in India': The Hindu

In India, police said they had arrested a 29-year-old man in the southern state of Kerala, close to Sri Lanka, for planning similar attacks there. The man had been influenced by speeches made by Zahran, the government's National Investigation Agency said in a statement.