Mahinda Rajapakse, interviewed on Monday, said: "There's only one country, we can share power. Not a separate state. That idea must be taken off ... it is completely out."
Sri Lanka is soon to hold talks with the rebels in Switzerland in an attempt to avoid a return to war.
The Liberation Tigers of Tamil Eelam (LTTE) have threatened to resume an armed struggle that killed more than 64,000 people during 20 years until a truce was struck in 2002 unless the government gives them a homeland for ethnic Tamils in the north and east of the country.
Rajapakse, who has trodden a moderate line since taking power in November, said he would meet a rebel demand to rein in armed groups. The Tigers accuse the military of helping a breakaway faction led by a renegade commander called Karuna to mount attacks, which the government denies.
"If any group operates in our area, we will stop it. Any groups carrying arms will be brought under control, whether it is the so-called Karuna group or the LTTE," Rajapakse said.
"There must be a [Tiger] guarantee too ... that civilians are not being harassed or not being killed."