U.S. President Barack Obama and British Prime Minister David Cameron vowed to continue their fight against the “distorted ideology” of Islamist extremists, in a joint opinion piece published Thursday in The Times of London. The joint op-ed comes ahead of Cameron’s two-day trip to Washington, which is likely to be his final trip to the U.S. before the UK's general elections in May.

“Whether we are facing lone fanatics or terrorist organizations such as al Qaeda, Islamic State or Boko Haram, we will not be cowed by extremists. We will defeat these barbaric killers and their distorted ideology,” Obama and Cameron wrote in the article, published just a week after terrorist attacks in Paris led the deaths of 17 people.

“Along with our French allies, we've made clear to those who think they can muzzle freedom of speech and expression with violence that our voices will only grow louder,” the two leaders wrote in the article titled, "We won’t let the voice of freedom be muzzled."

Obama and Cameron also condemned Russia’s aggression in Ukraine and accused Putin of “trying to destabilize” the region.

Western nations, including members of the European Union and the U.S., have imposed a series of economic sanctions on Russia in response to what they term an “illegal annexation of Crimea and deliberate destabilization of a neighboring sovereign country.” The Russian government has repeatedly condemned these sanctions and has threatened to impose counter-sanctions against the U.S. and the EU, according to media reports.  

“If we allow such fundamental breaches of international law to go unchecked, we will all suffer from the instability that would follow. Our strong and united response has sent an unmistakable message that the international community will not stand by as Russia attempts to destabilize Ukraine,” Cameron and Obama wrote.

According to a report by The Wall Street Journal, during his two-day trip to Washington, Cameron is expected to put pressure on Obama to criticize American social media companies like Facebook and Twitter for failing to provide access to encrypted communications.

Cameron has, in the aftermath of the Paris attacks, repeatedly spoken in favor of providing security agencies in the UK greater access to encrypted communications networks and has promised to introduce a legislation that would do so if his party secures a majority in the May elections.