"In some cases, foreigners of Sri Lankan origin may be detained without their embassies being notified," the state department said in its travel advisory to US citizens.
Sri Lanka has granted tourists a 30-day visa on arrival since the 1970s as part of a move to promote tourism, which was badly affected by the island's ethnic conflict which ended in May last year with the crushing of Tigers.
The local tourism industry has been buoyed by a near 50 percent increase in foreign visitors since the end of fighting last year.
Sri Lanka hopes to double the number of visiting tourists this year to 600,000.
The government had announced it was withdrawing the landing endorsements from September 30 in a move widely seen as potentially damaging to the island's tourism industry, which is recovering from 37 years of ethnic conflict.
"Notice is hereby given that the earlier public notice on 'Withdrawal of on-arrival visa facility' be treated as withdrawn until further notice," immigration co