LONDON—Tensions following Turkey's shooting down of a Russian jet are disrupting the Black Sea wheat trade, one of the world's biggest grain routes, in a spat that could push prices higher as some customers look for alternative supplies.

Some firms are deferring new deals to ship wheat from Russia to Turkey, citing fears Moscow may impose an export ban. Any disruption would come as a boon to the beleaguered grain industry in the U.S. and elsewhere in Europe, as those regions step in and tap the millions of tons of grain piling up in silos around the world.

No formal Russian ban on grain exports has been announced, but buyers have talked of delays getting customs' authorization for shipments to Turkey in the wake of the downed plane. After permitting ships to leave with grain cargoes Thursday, Russian authorities began suspending exports to Turkey on Friday, said one broker, who declined to be named.

On Tuesday, a Turkish jet fighter shot down a Russian bomber along the Syrian border, marking the first time a North Atlantic Treaty Organization member downed a Russian warplane since 1952. That led Russia to announce plans Thursday to impose sanctions against Turkey.

Wheat represents one of the main items traded between the nations, with Turkey the no. 1 destination for Russian wheat last year. Russia is the world's third-largest wheat exporter and, alongside Egypt, Turkey is its biggest customer. Turkey bought around 4.1 million metric tons of Russian wheat in the 2014-15 marketing year.

Last year, fears that Russia's involvement in a separatist war in Ukraine would hit the two countries' exports helped push up the price of wheat by nearly 25% over three months.

The market hasn't reacted to the current crisis yet, but a price spike is expected if the situation between the two countries worsens.

"If there is an escalation, I would expect that to give support to prices," said Daryna Kovalska, an analyst at Macquarie.

The muted market reaction, so far, is an indicator of the size of global wheat reserves, which gives the market a large cushion against disruption.

After three consecutive big harvests, this marketing year will end with stocks of about 209 million tons of wheat stockpiled, according to the International Grains Council. European wheat prices are down 12% this year.

Cheap grain from Russia and Ukraine also has hurt farmers in Europe and the U.S., with silos filling across major producers, such as France, as they struggle to shift their harvest.

Additional demand from Turkey, then, could comfortably be met by Russia's rivals, Ms. Kovalska said.

But if the supply of Russian wheat is cut off, there would be at least a short-term spike in prices, as Turkey's millers scramble to secure new supplies at short notice. It also would bolster sellers' bargaining power.

On Friday, Erol Yahya, executive director of Turkish milling group Intermil-Un, said his company had suspended new purchases of Russian wheat until the situation becomes clear. Intermil would have pay $30 to $40 more per metric ton for wheat from Canada, Australia or the U.S. if it were cut off from Russian supplies, he said.

European wheat futures for March delivery were trading around €183 per ton Friday.

Likewise, Russian trade houses have stopped striking new deals with Turkish buyers, fearing an export ban would leave their vessels stuck in port accruing fines, said Andrey Sizov, managing director of Russian agriculture consultancy SovEcon.

Many in the industry are hoping a long-term export ban won't happen, citing the countries' mutual dependence in trade. They hope the two governments will permit trade to continue quietly even as they spar in public.

Turkey was the no. 1 destination for Russian wheat, corn, sunflower oil, and sunflower meal in 2014-15, accounting for exports valued at about $1.5 billion, according to SovEcon's data. Meanwhile, Russia's boycott of some EU products has left it reliant on imports of fruit and vegetables from Turkey.

Write to Ed Ballard at ed.ballard@wsj.com

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(END) Dow Jones Newswires

November 27, 2015 13:35 ET (18:35 GMT)