Aboard A U.S. Military Aircraft (CNN) -- U.S. Defense Secretary Robert Gates said Sunday the Libyan operation is off to "a strong and successful start" but he warned against widening the goals of the mission beyond what was spelled out in the United Nations Security Council resolution.

He also brushed aside suggestions that there was a split inside the upper echelons of the Obama administration over whether to push forward with military action, saying there had been what he called unanimous agreement.

"Whatever positions people took in that debate, and in that discussion, there was unanimous support for the approach the president had decided on," Gates said on board the plane taking him on a previously scheduled trip to Russia. The defense secretary delayed his trip by a day just after the start of the Libyan offensive.

Gates has been a vocal opponent of the U.S. enforcing a no-fly zone, cautioning that it was a complicated, risky military operation.

He defended his comments in recent weeks, which seemed to signal his personal opposition to U.S. military participation.

"When I said, 'Let's call a spade a spade,' it was that a no-fly zone begins with an attack on Libya and I think that was a pretty accurate statement. So I think what I've tried to do is really just make clear what is involved in this and that it is a complex undertaking," Gates said.

He portrayed a full debate on how to proceed.

"Any president who is contemplating the use of military force should demand a spirited debate, an intense debate, among his advisers on all the ramifications," Gates said in a quick question and answer session just after his military jet lifted off from Andrews Air Force Base bound for St. Petersburg, Florida.

"And I would tell you that in terms of questions and uncertainties and outcomes there hasn't been a question asked -- publicly -- in the last 24 hours or so that wasn't discussed in depth in the lead-up to the launching of this operation."

Gates warned against widening the coalition operations to include a direct attack on Libyan leader Moammar Gadhafi.

Gates cautioned that anything that went beyond the U.N. Security Council resolution risked disrupting the "very diverse coalition" that agreed to the attacks on Libya.

"The one thing there is common agreement on is the terms set out in the Security Council resolution and if we start adding additional objectives then I think we create a problem in that respect," Gates said. "I also think that is unwise to set as specific goals things you may or may not be able to achieve."

The key is to establish a no-fly zone and "do what we can to prevent him (Gadhafi) from using his military forces to slaughter his own people," Gates explained.

The defense secretary said there are other options to assist opposition fighters beyond arming them, including pressuring the government with political and economic sanctions.

"Over the long term this basically is going to have to be resolved by the Libyans themselves," Gates said.