"We have seen six top-level US State Department officials visiting this country during the last four months," said Dallas Alahapperuma, a senior opposition leader and a loyalist of Rajapaksa.
"Why is there so much interest? We have reasons to be suspicious," he added.
Alahapperuma is a member of the Sri Lanka Freedom Party (SLFP) led by President Maithripala Sirisena.
The SLFP and Prime Minister Ranil Wickremesinghe's United National Party (UNP) form the current unity government.
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The US has markedly improved its engagement with Sirisena's government that took power in January this year.
Several high ranking US visits including that of Secretary of State John Kerry have taken place as part of the American policy of supporting the reconciliation and peace building in Sri Lanka after the end of the brutal three-decade civil war against the LTTE that killed about 100,000 people.
The US moved three anti-Sri Lanka resolutions in the UN human rights body during Rajapaksa's tenure calling for an international probe into alleged human rights violations during the final phase of the civil war that ended in 2009.