Economically beleaguered Ukraine earns an estimated $200 million a year simply by allowing foreign aircraft to cross its airspace enroute to other destinations.

The Convention on International Civil Aviation of 1944 allows nation​s to set "reasonable" charges for planes that  transit their airspace at altitude. Some nations are public about their charges. For example, rates to fly over the United States will increase on Oct. 1 to $56.86 per 100 nautical miles, up $6.91 over current rates. Air carriers will be charged less -- $21.63 -- to fly over a vast expanse of ocean controlled by the United States.

Other nations are less public about their overflight rates, and, in some cases, they are negotiated with individual carriers. The estimate that Ukraine earns $200 million came from an international aviation organization that keeps track of overflight charges.

After Malaysia Air Flight 17 apparently was shot down over Ukraine, some critics questioned whether the Kiev government allowed dangerous airspace to remain open so that it could continue to collect overflight fees. Ukraine had declared its airspace below 32,000 feet off limits to commercial flights. MH17 was reported at 33,000 feet when the incident occurred.

But a high-ranking official with the international aviation group, who asked that he and his organization not be identified because they were making available closely-held data, said Ukraine likely was under pressure from airlines to keep its skies open to overflights. The route taken by MH17 is popular for flights from European capitals to major cities in Asia.

"It's a complicated issue," the official said. "It's not just Ukraine that has an interest, it's every airline that flies there. They don't want to have to fly around [Ukraine]." Taking a less direct route would require burning more fuel, the number one expense for airlines.

"They were working on the assumption that the air is safe above a certain altitude," the official said. "They certainly don't want to put passengers at risk."

Since the plane went down on July 17, Malaysia Air has been routing the same flight -- Amsterdam to Kuala Lumpur -- well to the south of Ukraine. That has added an average of about 30 minutes to the flight, a period of time in which a Boeing 777 burns roughly 1,800 gallons of fuel. Based on the current cost of jet fuel in the United States, that would add in the neighborhood of $10,000 to Malaysia Air's costs.

U.S. airlines were advised by the Federal Aviation Administration not to fly over contested Crimea in April, and that prohibition was extended to adjacent regions of Ukraine after MH17 went down. The FAA moved swiftly on Tuesday to suspend U.S. flights to another conflicted region, Israel, after a rocket landed near the Tel Aviv airport.

"​Airlines work cooperatively with government authorities, including the FAA, which is in close contact with U.S. intelligence, [international agencies] and civil aviation authorities in other countries to assess risk and provide guidance to carriers accordingly," said Jean Medina, communications director for Airlines for America, a group that represents most of the nation's top airlines. "Information is shared on a frequent, and often daily basis."

Code sharing, which links U.S. carriers to foreign airlines, provides a wrinkle for those planning travel. A ticket sold by a U.S. airline may put a passenger on a foreign-flagged airline that is not bound by FAA restrictions on avoiding conflicted areas.

"Airlines do tell customers who is operating the flight at the point of booking," Medina pointed out.