He also rejected calls for another early parliamentary election, following the last one held in September.
"I understand that early elections are a live issue for some political forces, but I say a categorical no."
He also ruled out a "reorganisation of the coalition" and accused the government of "populism".
Analysts say the split between Ukraine's two most prominent politicians is worsening because both are likely to contest a presidential poll in 2009 or 2010.
Tymoshenko and other opposition groups have threatened constitutional amendments that would reduce the president's powers.
The two stood side by side in leading the 2004 Orange Revolution that peacefully overturned a rigged election originally awarded to a Moscow-backed candidate.
But they have frequently been at odds and Yushchenko sacked Tymoshenko when she first held the premiership in 2005.