Ukraine's President said he hoped United States President-elect Donald Trump would maintain his support for Kiev regarding the conflict with Russia.

Speaking alongside US Vice President Joe Biden, Petro Poroshenko said on Monday that Ukraine backed the democratic election of the US and was ready to cooperate with the new administration, Efe news reports.

"I hope the Ukrainian question continues to be a priority," Poroshenko said.

Russia had invaded Ukrainian territory, occupying part of Donbass in the east and Crimea, he said.

Incoming US Secretary of State Rex Tillerson told the US Senate Committee on Foreign Relations that Russia "has invaded Ukraine, including the taking of Crimea".

Before taking on the top diplomatic post, the former ExxonMobil chief had criticized Washington sanctions against Moscow.

Last week, Trump said he was willing to negotiate with Russia on lifting economic sanctions in exchange for the Kremlin agreeing to reduce its nuclear weapons stockpiles.

Meanwhile, Biden reiterated the US backing of political and economic reforms undertaken by the Ukrainian government in recent years.

The common good had to be placed above local interests, Biden said, adding that this applied to members of the government, the Supreme Council of Ukraine, judges, media and citizens.

Ukraine has been locked in a bloody conflict with a pro-Russian separatist insurgency in the east since February 2014.