WASHINGTON — The US and Russia have signed a memorandum of agreement about safe conduct while flying over Syria, but the ongoing mistrust between the two sides means the agreement may have little real-world impact.

The agreement, announced by the Pentagon's press secretary, Peter Cook, on Tuesday, includes a "series of protocols in place that effectively are intended to avoid any sort of risk of a midair incident between our aircrews and Russian aircrews.

"If they follow these protocols we should not have the risk of engagement of Russian aircrews over Syria," he said. "In the event there is an incident or engagement in the air, there are protocols they can follow and an ability to engage with each other in the air to make sure everyone is operating safely. As a back up to that, there is a separate line of communication that is available should communication in the air break down for some reason."

However, Cook said, the Russian government asked that the US not release the actual language of the agreement. Asked why the Russian government requested the details be kept secret, Cook said: "I know it was a request that was made and I'm not sure the full reason behind it."

As a result, details are scarce, with Cook largely dodging on specifics when asked by reporters.

What is known is that the agreement covers the US and its entire coalition operating in Syria, for both manned and unmanned systems. It also provides some guidelines on what frequencies pilots should us to communicate with each other if needed.

The biggest question which remains unsolved is the definition of "safe distance" between Russian and coalition pilots. Cook would not say if a specific distance was set in the agreement that should be maintained between vehicles. Russian jets have flown extremely close to US and coalition equipment in recent weeks, in some cases reportedly getting within hundreds of feet while midair.

It also is unclear what happens if either coalition or Russian pilots violate the terms of the agreement, with options likely limited by the same real-world factors that have allowed Russia to take control of large swaths of the Syrian skies — that the US is desperate to avoid a confrontation which could lead to a shooting war.

The agreement, reached through several weeks of negotiations between the Pentagon and Russian military officials, is built on shaky ground.

Cook acknowledged that there are trust issues between the two sides when he said that "the fact we had to resort to a memorandum of understanding to try and work out what should be standard protocol over Syria gives you an indication over our concern with Russia's activities."

And within minutes of Cook's briefing, the Russian Ministry of Defence released its own statement, saying it had requested an "agreement that is more substantial" than what the US agreed to.

"That is why a number of specific proposals aimed at deepening military cooperation between Russian and US militaries in countering international terrorism was put forward," a statement posted on the MoD's English language website read.

"For example, the American counterparts were requested to provide specific information concerning [Islamic State] targets in Syria. Russian MoD was ready after necessary crosschecking to launch air strikes against them. However, the response from Washington was negative," the statement said. "Russian negotiators also insisted on closer interaction in situations of aircraft shootdown or emergency landing. Joint aircrews' search and rescue operations were proposed. The idea was not supported either."

Cook, for his part, stressed that this agreement is a "narrow understanding on a specific issue" and does not mean the US will be coordinating with Russian forces in the future.

Twitter: @AaronMehta