WASHINGTON - US President Barack Obama on Monday hailed the second-round presidential elections set to begin in Liberia on Tuesday, vowing that the international community will hold accountable those "who choose to obstruct the democratic process."

"This historic vote is an opportunity for Liberians to strengthen the country's democracy, and to deepen its peace, prosperity and national unity," Obama said in a statement.

"We encourage all voters of all political preferences to exercise their universal right to participate in the democratic process, all Liberians should have the ability to exercise this right without fear of violence or intimidation," he added.

The first round of ballot on October 11, in which 16 parties participated, did not produce a clear winner. Fresh riot broke out on Monday in Monrovia, Liberia's capital, with casualties feared and properties being destroyed ahead of the run-off elections.

The United Nations has maintained a peacekeeping force in Liberia since 2003 to bolster a cease-fire agreement ending a decade of war that killed nearly 150,000 people, mostly civilians, and sent 850,000 others fleeing to neighboring countries.

"Liberia has taken important steps to consolidate its democracy since the end of its civil war. Those gains must not be setback by individuals who seek to disrupt the political process," Obama said.

"The international community will hold accountable those who choose to obstruct the democratic process," he added, noting that the US is "deeply committed" to the strength of democracy on the African continent.