US Secretary of State Rex Tillerson has warned Russia that its interventions in Ukraine are an obstacle to improving relations between the two nations, a media report said.

Urging both Russia and pro-separatists in the Ukraine to immediately respect the ceasefire, Tillerson made the remarks in a phone call with Ukrainian President Petro Poroshenko on Sunday, Efe news reported. 

During their conversation, Tillerson also talked about the results of his visit to Moscow earlier in April.

According to a statement by US Department of State spokesperson Mark Toner, both leaders talked about the trip and the message that Tillerson sent to the Russian government earlier.

The statement said that although the US was interested in improving relations with Russia, but President Vladimir Putin's government's actions in the east of Ukraine remained an obstacle.

Tillerson also assured the US government's firm commitment to supporting the territorial integrity and sovereignty of Ukraine.

He said that sanctions against Moscow would "remain in place until Russia returns control of the Crimean peninsula to Ukraine and fully implements its commitments in the Minsk agreements."

The agreements signed in Belarus capital Minsk in February 2015, aimed to put an end to the armed conflict between the Ukrainian government and the pro-Russian separatists and to support the country's territorial integrity.

The Minsk agreement initiated a settlement process which despite making considerable progress, has failed to end the hostilities, as there were still violations of the ceasefire in the area.

Tillerson's remarks on sanctions against Russia were a change in the stance of the US government regarding the conflict, as President Donald Trump suggested in January that he would end the sanctions against Moscow for its interference in Ukraine, an idea that France and Germany opposed.

During the phone call, Poroshenko also conveyed his condolences to the US government for the recent passing away of an American observer for the Organization for Security and Cooperation in Europe's monitoring team (OSCE).

The OSCE observer lost his life on Sunday in a landmine explosion in the separatist province of Luhansk in eastern Ukraine.

Both Tillerson and Poroshenko agreed that this tragic incident highlights the need for all parties, especially those led by pro-Russian separatists, to immediately comply with the Minsk agreements.

The US State Department also extended its condolences to the family and friends of the victim and condemned the incident in another statement.

"This death underscores the increasingly dangerous conditions under which these courageous monitors work, including access restrictions, threats and harassment," the statement said. 

"The US urges Russia to use its influence with the separatists to allow the OSCE to conduct a full, transparent and timely investigation," the US Department of State statement said.

Poroshenko and German Chancellor Angela Merkel have already demanded an investigation to clarify the death of the observer.