ST. PETERSBURG, Russia — Russia's foreign minister said Friday that Washington is sending contradictory signals on Syria that could derail an international conference intended to end that country's civil war, warning that U.S. talk about a possible no-fly zone would only encourage the rebels to keep fighting.

Sergey Lavrov also criticized demands that Syrian President Bashar Assad step down.

"Not because we like the regime, not because we want the regime to stay, but because it's for the Syrians to decide," Lavrov said in an interview. "And to say you must capitulate and deliver the power to us is just not realistic."

In addition, he dismissed allegations by the U.S., Britain and France that Assad's regime has used chemical weapons.

Russia has been the key ally of Assad's regime throughout the Syrian conflict, which has killed more than 93,000 people, shielding it from U.N. Security Council sanctions and continuing to provide it with weapons.

Russian President Vladimir Putin has confirmed that his country signed a contract for the delivery of S-300 state-of-the art air defense missile systems to Syria, but said it hasn't been fulfilled yet.

Lavrov defended the S-300 deal, pointing to the deployment of U.S. Patriot air defense missiles and fighter jets to neighboring Jordan.

"The contract on S-300s is absolutely legal, it's transparent and it's fully in line with the international norms and with the Russian export control legislation," he told The Associated Press and the Bloomberg news agency.

"Second, the contract hasn't been yet finalized. Third, the Americans are leaving Patriots after this exercise in Jordan, together with F-16 planes, and no one is asking them not to do this. The region is really full of weapons, including offensive weapons which have been supplied in the past to the countries of the region, and some of these weapons are infiltrating into Syria."

He said supplies of weapons to the Syrian opposition, which have been promised by Washington and are being considered by the European Union, would be a "very big mistake."

Lavrov was asked if Russia is warning the West in particular against providing the rebels with shoulder-fired air defense missiles that could challenge Assad's air dominance. He replied: "We believe this is absolutely illegitimate, and we aren't going to legitimize it by starting discussing some conditions on which these supplies could be justified."

Putin, speaking to reporters after meeting with German Chancellor Angela Merkel at a business forum in St. Petersburg, argued that Western weapons, if they are supplied to the opposition, could fall into the hands of al-Qaida-linked militants who form part of the rebel force and eventually be used against the West.

Putin also warned that if Assad steps down, that could lead to the vacuum of power that would be quickly filled by al-Qaida-connected rebels. "How can we avoid it? There are well-armed, and very aggressive," he said.

Lavrov said during Friday's interview that while the U.S. says it favors sponsoring a Syria peace conference in Geneva, it has made statements that have sent a conflicting signal to the rebels. Lavrov said that the U.S. talk about a possible no-fly zone in particular has encouraged the opposition to step up fighting instead of sitting down for talks.

"The message the opposition is getting: Guys, don't go to Geneva, don't say you are going to negotiate with the regime, soon things will change in your favor," Lavrov said. "It's either the conference or the instigation of the opposition not to be flexible. I don't think it's possible to do both at the same time."

The date and location of the international conference on Syria haven't been announced yet, but it's already being dubbed "Geneva 2" because a similar event was held there a year ago.

"If our goal is the conference, then we must avoid any discussions and, of course, any action designed to establish a no-fly zone. We must avoid confrontational debates and one-sided resolutions in the (United Nations) General Assembly and in the Human Rights Council because all this isn't helping to create the atmosphere necessary to convene a conference," Lavrov said.

He shrugged off the U.S., British and French statements about the use of chemical weapons by the Syrian regime as ungrounded, saying they "smack of politics and speculation."

"We have been told by the Americans, by the French, by the British that they have proofs," he said. "What they showed to us is absolutely unconvincing. It's not based on facts, and it can't be taken as a proof."

He said that a new international probe must determine the truth and added that after the conflict is over, Syria could be encouraged to destroy its chemical weapons stockpiles.