Ambassador Teplitz said that as the investigation into Sunday's ISIS-claimed attack continued, a team of both FBI agents and USA. military personnel were in Sri Lanka assisting the probe.
Islamic State claimed responsibility for the attacks via its AMAQ news agency.
Experts say the bombings bear numerous hallmarks of IS attacks, and the government has suggested local militants could not have acted alone.
The announcement comes after the death toll rose again to 359 on Wednesday with more than 500 people wounded.
He was one of nine terrorists who carried out a series of bombings in churches and hotels across the Easter weekend.
Last night, Daesh said in a statement: 'Those who carried out the attack that targeted nationals of the coalition and Christians in Sri Lanka the day before yesterday are Islamic State fighters'.
A Sri Lankan security dragnet hunting those responsible for horrifying bombings that claimed more than 350 lives has scooped up a further 18 suspects, police said Wednesday, April 24, as pressure mounted on politicians to explain why no one acted on intelligence warnings. She said a team of FBI agents and US military officials were helping in the investigation.
It was not clear if Sirisena was aware of the report but the top security organisation, the Security Council, reports to him, while the prime minister was no longer invited to council meetings because of the rift, Senaratne said.
Junior minister Ruwan Wijewardene also told reporters that numerous bombers were highly educated and came from well-off families.
Nearly 10 years after the end of a brutal civil war between the Tamil Tiger separatist group and the government, Sri Lanka has been plunged into violence yet again.
Benjamin Netanyahu wants to name Israeli town after Trump
"The Israeli government is not a racist government, nor does it include a single racist minister", he said. I just believe that the United States should deal with the Middle East on a level playing field basis.
Although there was intelligence report about these attacks, he was not informed about them, he said.
"The restructuring of the security forces and the police will be completed within a week", he said.
The terrorist group ISIS has claimed responsibility for the Sunday's attacks.
Sri Lanka's state minister for defence Ruwan Wijewardene today told parliament the bombings were a direct response to the March 15 attacks on two mosques in the New Zealand city.
An Australian white supremacist was arrested in the Christchurch shootings.
Speaking at a news conference on Tuesday, the prime minister said: "We could have prevented these attacks - or at least reduced the number of attacks".
The vast majority of those killed in the blasts were Sri Lankan, many from the island nation's Christian minority.
On Wednesday, the U.S. envoy to Sri Lanka warned that there were "ongoing terrorist plots" in the country. The ninth suicide bomber has not been identified, though two more suspects were killed in a later explosion on the outskirts of Colombo.
During the discussion, the Ambassadors and the High Commissioners of all countries had pledged to render their assistance to Sri Lanka in the aftermath of last Sunday's attacks.