UKRAINE (CNN) — US Defense Secretary James Mattis vowed continued US support for Ukraine in the face of Russian “aggression” as he met with the country’s leaders in Kiev on Thursday.

Mattis attended a Ukranian Independence Day parade in the capital before sitting down with his Ukrainian counterpart, Defense Minister Stepan Poltorak, and President Petro Poroshenko.

Speaking alongside Poroshenko at a news conference, Mattis said he intended to strengthen the US relationship with Ukraine in the face of Russia’s annexation of Crimea in 2014 and the ongoing conflict with pro-Russian separatists in eastern Ukraine.

“Have no doubt, the United States stands with Ukraine. We support you in the face of threats to sovereignty and territorial integrity, to international law, and to the international order writ large,” he said.

“We do not, and we will not, accept Russia’s seizure of Crimea and despite Russia’s denials, we know they are seeking to redraw international borders by force, undermining the sovereign and free nations of Europe.”

Mattis said the United States would continue to pressure Moscow to live up to its commitments under the 2014 Minsk agreement, saying Russia had “put its reputation on the line” when it signed up to the deal, never fully implemented.

“The US will continue to press Russia to honor its Minsk commitments and our sanctions will remain in place until Moscow reverses the actions that triggered them,” he said.

The Minsk agreement calls for an immediate ceasefire, withdrawal of all heavy weapons and unfettered access to monitors from the Organization for Security and Cooperation in Europe to Ukraine’s Donbas area, which takes in the Luhansk and Donetsk regions. It was negotiated by the leaders of the so-called Normandy Four — Ukraine, Russia, France and Germany.

“We in the United States understand the strategic challenges associated with Russian aggression — alongside our allies, we remain committed to upholding the widely accepted international norms that have increased global stability for decades,” Mattis added

Mattis said he would go back to Washington with a clearer idea of the needs of Ukrainian soldiers on the front line.

A key issue is whether the United States should provide Ukraine with defensive armaments, such as anti-tank missiles, in addition to non-lethal military equipment.

The United States has approved the provision of nearly $750 million-worth of military equipment in recent years, Mattis said.

A new ceasefire is supposed to come into force on Thursday, Poroshenko said, to coincide with the start of a new school year.

Asked by a reporter what steps he would now like to see, Poroshenko urged Russia immediately to withdraw its troops from the Donbas region. Ukrainian assessments indicate that there are currently about 3,000 members of the regular Russian forces on Ukrainian territory, he said.

He described the Russian military presence in his country as “extremely dangerous” and called for the troops’ immediate withdrawal, as well as a halt to the flow of new weapons into the area, the release of hostages and full access for OSCE monitors to areas occupied by pro-Russian forces.

Poroshenko also raised the idea of “the possible presence of the UN peacekeepers” in the region in order to protect a ceasefire between Russia and Ukraine.

The Ukrainian president, who met with US President Donald Trump at the White House in June, also expressed his gratitude for America’s ongoing support.

The Kremlin said Tuesday that the leaders of Russia, Ukraine, France and Germany “strongly supported” the decision by negotiators to announce the latest ceasefire.

“The leaders expressed hope that such a ceasefire would lead to a sustainable improvement in the security situation for the benefit of schoolchildren and the entire civilian population of Donbas,” said a statement released by the Kremlin after a phone call between the leaders of the Normandy Four.

The leaders pledged to continue working together for the further implementation of the Minsk commitments for a peaceful solution to the crisis, the statement said.

Western leaders and Kiev have long accused Russia of fostering the conflict by providing weapons and training to the pro-Russian separatists, as well as sending regular Russian troops over the border to fight. Moscow has denied the allegations.

The-CNN-Wire™ & © 2017 Cable News Network, Inc., a Time Warner Company. All rights reserved. (Photo: CNN)