JAKARTA, March 13 (Reuters) - Authorities in Indonesia's popular holiday island of Bali have asked the central immigration agency to cancel a visa-on-arrival policy for Russian and Ukrainian nationals after tourists were found working illegally.

Russian nationals are among the biggest groups of foreign arrivals in Indonesia, according to official tourism data. At least four Russian citizens were deported this month for visa violations and immigration authorities have repeatedly warned foreigners in Bali against working on tourist visas.

Bali Governor I Wayan Koster said on Sunday he has asked the Ministry of Law and Human Rights to tighten visa requirements by cancelling the visa-on-arrival facility specifically for Russian and Ukrainian citizens due to a series of violations.

"Because they are at war, so they flocked to Bali, including those who sought comfort or came to work here," he said, according to a post on the immigration agency's Instagram account. He added these two countries were singled out because their citizens' infractions were more "significant" than those of others