German Chancellor Angela Merkel has said it is "too early to say exactly how we will proceed" in forming a government.
If her current coalition lacks a majority and her party cannot govern alone, the likeliest outcome is a Merkel-led alliance with the Social Democrats, the Associated Press reported.
The two are traditionally rivals, but governed Germany together in Merkel's first term after an inconclusive 2005 election.
"The ball is in Merkel's court," her center-left challenger, Peer Steinbruck, said. "She has to get herself a majority."
More top news
Rain across much of the remainder of the country
Singer knocks Oasis from their 18-year hold on the top spot as her third album sells more than 800,000 copies in its first week of release
Francois Hollande has urged British MPs to support David Cameron's plan to launch airstrikes against so-called Islamic State in Syria.