(New throughout, adds analyst comment, detail)
By Helen Murphy and Nelson Bocanegra
BOGOTA, Sept 26 (Reuters) - Colombia's central bank on Friday held the benchmark interest rate steady for the first time in six months, as expected, hoping to bolster economic growth in an uncertain global economy and with domestic inflation under control.
The seven-member board voted to keep the lending rate at 4.5 percent, meeting expectations of 29 of 36 analysts in a Reuters poll. The decision to keep the rate at its highest level since October 2012 was not unanimous.
The bank statement highlighted the contrast between economies that could impact Colombia. A recovery in the United States comes as China, Europe and other Latin American nations are slowing, and trade terms could deteriorate along with a drop in international oil prices, it said.
"The uncertainty of not knowing how strong the global economy is and how it could impact Colombia is, I think, what pushed them to hold the rate," said Camilo Perez, chief economist at the Banco de Bogota.
Policymakers also decided on Friday to extend the bank's dollar purchase program through December, but reduced the level to up to $1 billion from $2 billion. It accumulates reserves to limit the impact of external shocks.
The bank began a tightening cycle in April, lifting the rate from 3.25 percent, where it had remained for 11 months. The increase was meant to stem any future inflationary pressures.
Colombia's economy expanded less than expected in the second quarter, slowing to 4.3 percent from 4.5 percent a year earlier and shrinking compared with the first three months of the year.
The monetary authority forecast second half economic growth would be about 4.5 percent, slower than 5.4 percent in the first half.
"Aggregate demand continues to show strong growth in a context near to full utilization of productive capacity," the bank statement said.
"At this time, inflation expectations remain around 3 percent. This occurs in an environment of deteriorating terms of trade and increased uncertainty about the recovery of the world economy and the cost of external financing."
Policymakers have expressed concern in recent months about Colombia's revenue stream as crude output begins to decline and an economy-driving oil boom wanes.
"The comment on the strong behavior of domestic demand is the reason why the decision was taken by majority and not unanimous, as it is likely that for some members of the board this was evidence the output gap is closed," Citibank economist Munir Jalil said in a note to investors.
"We believe at this level of the repo rate the upcoming decisions on the monetary policy front will become more data dependent," he said.
The economy is likely to grow this year to around 5 percent, the bank has said, versus expansion of 4.7 percent in 2013. First-quarter growth was 6.4 percent.
Colombia's inflation is forecast to end the year slightly higher than the mid-point of the 2- to 4-percent target range, a still-comfortable level, but above a near 50-year low of 1.94 percent in 2013. (Additional reporting by Julia Symmes Cobb and Luis Jaime Acosta in Bogota; Editing by Chizu Nomiyama and David Gregorio)