"Vietnam strongly protests the fact that Khmer Kampuchea Krom activists held an illegal protest and set a Vietnamese flag on fire in Cambodia's capital Phnom Penh on Tuesday," said Vietnam's Ministry of Foreign Affairs (MOFA) spokesperson Le Hai Binh on Wednesday.

"These unruly activities are deliberate to seriously insult feelings of Vietnamese people, inappropriate with the good traditional neighboring friendship between Vietnam and Cambodia," said Binh.

"Vietnam urges Cambodia to strictly handle these activities following law and have effective measures to prevent the similar incident from happening again," said Binh in a press release on MOFA's website on Wednesday.

Cambodia's ethnic minority Khmer Krom monks and activists set a Vietnamese flag on fire in Phnom Penh on Tuesday afternoon, as a protest to demand a Vietnamese diplomat to recognize the "true history" of the former Kampuchea Krom provinces entered the second day, drawing nearly 200 protesters rallying outside the Vietnamese Embassy.