Security officials in Sri Lanka have warned that the Islamist militants behind the Easter Sunday suicide bombings are planning more attacks. 

They said there was a risk of imminent attacks in five locations involving attackers dressed in military uniforms. 

"There could be another wave of attacks," the head of ministerial security division (MSD), a unit of the police, said in a letter to lawmakers and other security sections, seen by Reuters on Monday.

"The relevant information further notes that persons dressed in military uniforms and using a van could be involved in the attacks." 

Speaking to reporters, the Archbishop of Colombo, Cardinal Malcolm Ranjith, claimed to have seen an internal security document that warned of more attacks on churches.

The Catholic Church suspended all masses across the island as a precaution while Anglican churches were told by bishops to "prayerfully discern" whether to go ahead with their services. 

Some Christians chose to observe Sunday worship in their homes.  

The Archbishop of Colombo, Cardinal Malcolm Ranjith, delivered a televised sermon from a chapel at his home in which he called for unity. 

"We cannot kill someone in the name of God... It is a great tragedy that happened," the Archbishop said in his sermon.

"We extend our hand of friendship and fraternity to all our brothers and sisters of whatever class, society or religion that differentiates us."

The service was attended by President Maithripala Sirisena, Prime Minister Ranil Wickremesinghe and former President Mahinda Rajapaksa.

Sunday passed off without any attacks and the government lifted a curfew in the evening that had been in place since the attacks on churches and hotels last week that killed 253 people.

Security across the island has been ramped up after the attacks, which the Islamic State has claimed responsibility for, and last week police carried out a number of deadly raids on suspected militant safe houses. 

On Saturday, President Sirisena outlawed the militant groups National Thawheedh Jamaath (NTJ) and Jamathei Millathu Ibrahim, which are suspected of carrying out the attacks. 

Christians have spoken of their forgiveness for the attackers and asked for prayers for comfort and healing, as well as unity among the many faith and ethnic groups in the country.