KIEV, Sept 18 - Ukraine raised $3 billion on Monday in its first sovereign bond issue since restructuring its debt in 2015, drawing widespread demand that one source said reached three times that sum, Reuters reported.

President Petro Poroshenko said the success of the 15-year dollar placement showed investors had confidence in Ukraine's reform program, though a former economy minister cautioned it might encourage authorities to dilute those plans.

Capitalizing on a revival of investor appetite for high-yield assets, the new issue and a concurrent debt-swap operation were designed by the finance ministry to lighten the debt-servicing burden.

Finance Minister Oleksandr Danylyuk said the Eurobond was placed with a yield of 7.375 percent.

"Never before has Ukraine raised such a sum, never has it raised it under a 15-year maturity. This is an unbelievably positive assessment by investors of the reform (program)," Poroshenko was quoted as saying by news agency Interfax Ukraine.

A source close to the deal said nearly 400 investors participated in the issuance with orders of around $10 billion.

Returning to the international financial market by selling hard-currency bonds is part of the $17.5 billion aid-for-reforms program Ukraine agreed with the International Monetary Fund in 2015.

Some pro-reform activists and politicians worry that a successful issuance could make authorities less inclined to meet the IMF's demands, which include pension reform and a crackdown on corruption.

"If we get all the money from the international financial markets, there would be less need for the leadership of the country to rely on the IMF," ex-economy minister Aivaras Abromavicius told Reuters ahead of the placement.

"That unfortunately means that perhaps there would be the possibility of a slowdown on the reform path."

Of the $3 billion raised on Monday, $1.7 billion will go on the debt swap. The remaining $1.3 billion is earmarked for fresh financing, the source said.

Earlier in the day, Ukraine said it had agreed to buy back more than $1.5 billion of bonds due in 2019 and 2020 - in line with the amount it targeted to purchase through the debt swap.

In an announcement published on the Irish Stock Exchange, the Finance Ministry said it had accepted $1.16 billion on its 2019 bonds and $415.15 million on those due in 2020 in exchange for longer-maturity bonds.

This eases painful debt commitments in 2019, a year when potentially disruptive parliamentary and presidential elections are due.

"For the government the most important task is to lower the (debt) burden for 2019 when the electoral cycle will hit a peak and the opportunity to attract financing will significantly decrease," Olena Bilan, chief economist at Ukrainian investment bank Dragon Capital, said ahead of the swap.

Before the swap, Ukraine's external debt payments, including on quasi-sovereign Eurobonds -- issued by state-owned companies - were due to hit a peak of $7.5 billion in 2019 with a further $6.1 billion due in 2020, Bilan said. (rt/ez)