Tensions continue to rise between Turkey and Russia as both sides increase their military power at the Turkish-Syrian border.

Accusations between the two countries grew after Turkey shot down a Russian military jet last week.

Turkey said Russia invaded Turkish airspace illegally after being warned to fly out.

Since then, the two world leaders have thrown complaints and charges at each other. Russian President Vladimir Putin accused Turkey of aiding Islamic State oil smuggling. Turkish President Recep Tayyip Erdogan challenged Putin to prove it.

Erdogan said he would resign if Russia could prove the charge. Russia offered no evidence to support the smuggling claim.

Russia's Deputy Defense Minister Anatoly Antonov told journalists in Moscow this week that Turkey is the main destination for the oil stolen from its legitimate owners, which are Syria and Iraq. Turkey resells this oil. He said the appalling part about it is that the country's top political leadership is involved in the illegal business — President Erdogan and his family.

Charges that Turkey profits from buying cheap oil from smugglers is a sensitive topic. Erdogan's son, Bilal, has been named in the issue. The Turkish president denied his son's involvement and any wrong doing.

On the Syrian border with Turkey, military forces were added this week. Russia announced that its Su-34 bombers that are flying over Syria now carry air-to-air missiles for self-defense.

The Turkish air force increased its patrols along the border.

Tuesday, Turkey called for the two countries "to gather around the table and talk." Turkey added it would talk but would also continue to protect its borders and airspace.

In another developing story on Wednesday, NATO foreign ministers invited Montenegro Wednesday to join the military alliance.

It would be NATO's first addition since 2009. It also defies Russian warnings that Balkan countries joining NATO is "irresponsible."

Montenegro is an ex-Communist country on the Adriatic Sea. It borders Serbia and Bosnia-Herzegovina. Montenegro plans on becoming a full NATO member in 2016.

I'm Anne Ball.

Dorian Jones wrote this story for VOAnews.com. Jim Dresbach adapted it for Learning English. Kathleen Struck was the editor.