The United States and Ukraine on Tuesday vowed that they would not recognize Russia's “illegal” annexation of Crimea, as the international community looks to ramp up pressure on the Kremlin.

“Ukraine and the United States emphasize that they will not recognize Russia's illegal attempt to annex Crimea,” said the nations in a joint statement after talks at the Nuclear Security Summit in The Hague.

“Crimea is an integral part of Ukraine. The United States will continue to help Ukraine affirm its sovereignty and territorial integrity. As the people of Ukraine work to restore unity, peace, and security to their country, the United States will stand by their side,” the statement added.

The nuclear summit in the Netherlands has been overshadowed by the standoff with Russia over its takeover of Ukraine's Crimean peninsula and has become a key test of President Obama's foreign policy.

On Monday, the Group of Seven announced that they were suspending Moscow from the G-8 and had cancelled a planned meeting in Sochi, Russia -- instead making plans to meet separately in Brussels.

The U.S. and European allies have warned Russia not to make further incursions on Ukrainian territory and to allow international monitors in Crimea and begin walks over the disputed area with Kiev. But Russian President Vladimir Putin has shown no signs of backing down, with reports Monday saying that the Kremlin was massing troops near Ukraine's borders in another show of strength.

The U.S. and European Union have enacted targeted sanctions against Russian and Ukrainian officials behind the annexation of Crimea and have vowed to put in place tougher measures targeting Russia's economy if Putin does not stand down.

But finding a consensus on further sanctions will prove difficult for Obama, with European leaders, who are heavily dependent on Russia for energy, fearing economic blowback.

The Senate on Monday advanced an aid package for Ukraine, but that bill faces an uncertain future in the GOP-controlled House, where conservative lawmakers oppose measures that would institute reforms to the International Monetary Fund.

Opponents say those measures would weaken U.S. voting power at the IMF, but the Obama administration says passage is essential to help the organization more effectively provide aid to Ukraine.

Following the break-up of the Soviet Union, Ukraine dismantled its nuclear arsenal in exchange for security guarantees with both the U.S. and Russia. The U.S. said that Russia’s takeover of Crimea had broken those accords and pledged to work with Kiev to further their commitment to nuclear nonproliferation.

The U.S. is supporting the construction of a nuclear research facility in Ukraine and will also provide additional resources to protect sites with radioactive material and track nuclear materials.