LONDON (Reuters) ? Political trends across Latin America are making international firms anxious about doing business in the region, a leading insurance brokerage said on Tuesday.

"The emergence of more leftwing governments in Latin America, including Venezuela and Bolivia, is causing concern for foreign business," said Charles Keville, a director in the crisis management team of Aon Corp., the world's second-largest insurance brokerage.

Aon has increased its risk rating on Bolivia to "high risk" in its latest political and economic risks map, published on Tuesday in London.

Aon's move followed Bolivia's decision to impose a 50 percent royalty and tax charge on hydrocarbons, substances used to make fuels and chemicals, which Aon said could result in foreign companies withdrawing from the country.

Venezuela, which has seized land from foreign firms and has spoken of changing oil and mining contracts with foreign companies, is also classed as being high risk by Aon, while Nicaragua and Colombia, both important for foreign trade, are classified as "medium-high risk."

Foreign investors and political risk insurers -- who cover firms against a range of risks involved in investing overseas, such as non-payment of contracts or asset expropriation by governments -- are keeping a close eye on the presidential election in Mexico, said Keville.

This is because it is uncertain whether the election of leftist candidate Manuel Lopez Obrador, who is the frontunner in the campaign for Mexico's July 2 election, may make the country a more hostile environment for foreign investors, said Keville.

In more positive news for the region, Brazil has been upgraded by Aon to "medium-low risk" as fears of its economic collapse have been alleviated. "The risks in Brazil have lessened recently," said Keville. Insurers are also starting to offer more coverage for risks in Argentina, too, he added.

RUSSIA

The gas crisis that hit central and eastern Europe over Christmas caused by a pipeline dispute between Russia and Ukraine is unlikely to prompt insurers to put up their rates for cover in the region, said Keville.

"I haven't had time to poll a lot of underwriters, but I would say no, it won't affect rates," said Keville. "After all, the risks associated with pumping oil and gas in the region aren't exactly new."

But foreign firms are fearful of increasing political interference in Russia, he said.

"There is investor nervousness regarding the sanctity of contracts," said Kim Hills, an analyst at Oxford Analytica, the international consulting firm that helped Aon draw up the risk map.

The map rates countries according to the amount of capacity offered by London insurance underwriters to foreign firms seeking to do business in those countries to cover against a range of political and economic risks.