US Assistant Secretary of State Nisha Biswal met with Tamil National Alliance (TNA) leaders as part of her two-day visit to Sri Lanka.
Lawmaker M A Sumanthiran said the party had hoped the government would release some prisoners before the country's independence day tomorrow and would quickly hand back land to its owners.
The surprise election victory of President Maithripala Sirisena last month and his government's early steps to end repression have stirred hopes in Washington that Sri Lanka and the US can revive strained ties.
Sirisena's government has promised to take steps to achieve reconciliation between minority Tamils and majority Sinhalese.
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"We see some lethargy, some delay. We told the US to encourage the government to (move) fairly quickly," Sumanthiran told AP.
Sumanthiran also said his party discussed power sharing as a political solution to the long ethnic conflict on the island.
Tamil leaders have accused the government of former President Mahinda Rajapaksa of reneging on its promise to offer a degree of self-rule after the civil war.
US-Sri Lanka relations were strained over Rajapaksa's reluctance to investigate thousands of reported civilian deaths in the final months of the conflict in 2009, when government forces crushed Tamil rebels who had been fighting for an ethnic homeland.
A United Nations report said earlier up to 40,000 civilians may have been killed in the last months. Accurate estimates of the number of deaths in the 26-year war are not available.
The US sponsored a resolution at the UN Human Rights Council last year calling for an investigation into war crimes allegations, and a report from the probe is scheduled to be released in March.