BOGOTA, Feb 27 (Reuters) - Colombia plans to buy new helicopters and aircraft and send more troops to counter rebels after approving a $3.7 billion, four-year investment plan to upgrade its military, the government said on Tuesday.

Defense Minister Juan Manuel Santos said the government would purchase fighter aircraft and helicopters and put 38,000 more police officers and soldiers on patrol to consolidate a U.S.-backed crackdown on Marxist rebels.

Washington has funneled around $4 billion in mainly military aid to Colombia since 2000, helping key ally President Alvaro Uribe battle the drug trade that partly fuels Latin America's oldest guerrilla war.

"We are not going to enter into an arms race with any of our neighbors," Santos told local radio. "This simply maintains a strategic base, bolsters troops on the ground and our capacity to consolidate the policy of democratic security."

Neighboring Venezuela, whose left-wing President Hugo Chavez is increasingly at odds with the United States, has spent billions of dollars of its oil revenues on arms purchases including Russian attack helicopters, fighter jets and automatic rifles.

Santos said much of Colombia's new military spending would come from a special tax the government proposed last year on wealthy Colombians and companies.

Under the investment plan, Colombia's police force will expand by around 20,000 officers with investment in better transport and intelligence equipment while aging naval frigates and submarines will also get upgrades.

Uribe, reelected in August last year, remains popular for security policies that have pushed back the FARC rebels from urban areas and highways and disarmed right-wing militias who once battled the guerrillas.

The U.S. government is asking Congress to approve another $3.9 billion over seven years for Colombia, but some Democrats are concerned about a scandal tying some of Uribe's political allies to illegal right-wing militias accused of atrocities.

Eight pro-Uribe lawmakers and his former security police chief have been arrested on charges they aided or helped organize paramilitaries. Rights groups say militia commanders have been allowed to keep crime gangs intact even after entering into a peace deal with Uribe.

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