By Susan Cornwell

WASHINGTON (Reuters) - U.S. Senate Democrats would not agree to postponing completion of this year's spending legislation until 2017, the chamber's top Democrat said on Thursday, warning the majority Republicans against considering such an approach when Congress returns next week.

Senate Minority Leader Harry Reid spoke after some conservative groups outside Congress earlier this week said it would be better for lawmakers not to finish a government appropriations bills until early 2017, when there will be a new president who could work out a deal with a newly elected Congress.

The lawmakers return to work next week from a lengthy recess with appropriations bills unfinished for the coming fiscal year, which starts Oct. 1. By law, lawmakers must act by Sept. 30 to approve the spending bills, or the government shuts down, as it did for two weeks in 2013 when appropriations bills had not been enacted in time.

In a conference call with reporters, Reid said he worried that "we're headed straight for another unnecessary government shutdown" unless action is taken.

Congress could pass a stop-gap continuing resolution that would keep the government running at current spending levels until next year, postponing final appropriations decisions until then without shutting down the government.

Reid said Democrats favored approving a shorter stop-gap bill that would let lawmakers return to Washington later this year - presumably after the Nov. 8 election - to finish spending bills.

"But we are not doing anything into next year," he said. "And the Republicans should be made aware of that right now."

Reid said he had met with President Barack Obama on Wednesday, implying Obama endorsed his stance.

Although Democrats are in the minority in both the Senate and House, rules requiring a supermajority for many Senate actions give Democrats significant leverage.

More than two dozen conservative groups wrote to lawmakers this week saying they were worried that Congress would use what is called a "lame duck" session immediately after the election to tack on additional spending and corporate welfare provisions to the appropriations bills.

Among other things, Congress has not been able to decide how much to spend to fight the mosquito-borne Zika virus.

The Republican leadership has not announced its plans for finishing work on spending bills.

Staff members for both Republican leaders in the House and Senate said they would discuss the issue next week.

(Reporting by Susan Cornwell; Editing by Julia Edwards and Bill Trott)