Just before a critical NATO summit meeting today in Wales, the leaders of the U.S. and U.K. accused Russia of having "ripped up the rulebook" in regards to Crimea and Ukraine, in a dual-signed letter that appeared in The Times of London.

President Barack Obama and Prime Minister David Cameron said that NATO needed to strengthen its alliance to "use our military to ensure a persistent presence in eastern Europe" and ensure a strong message is sent to Russian President Vladmir Putin that the Western allies are not going to back down, NBC reported.

"To the east, Russia has ripped up the rulebook with its illegal, self-declared annexation of Crimea and its troops on Ukrainian soil threatening a sovereign nation state. To the south, there is an arc of instability from North Africa and the Sahel to the Middle East," Obama and Cameron wrote.

In addition to the ongoing tensions in eastern Europe, the two leaders also sent a strong message to the radical militants of the Islamic State.

In the letter, they called the execution of two American journalists "the latest evidence of a brutal and poisonous extremism that murders indiscriminately and risks exporting terrorism abroad."

The two vowed to commit to spending 2 percent of their respective country's GDPs on defense, The Hindu reported.

Early Thursday, Cameron told BBC he would not rule out airstrikes, joining the U.S., if it was in line with pursuing national interests, The Hindu reported.

French President Francois Hollande has also spoken on the need for appropriate responses to the issues.

"We mustn't see this as something where you have a Western intervention over the heads of neighboring states and leaving others to pick up the pieces," he said.

Obama arrived in Wales Wednesday night and stopped in Estonia on the way to speak to reporters.

NATO is "poised to do more to help Ukraine strengthen its forces and defend their country," he said, according to NBC.