WASHINGTON (AFP) - President Barack Obama welcomed Germany's Angela Merkel to the White House on Friday, seeking to secure united European backing for tougher sanctions on Russia's economy should the Kremlin escalate the crisis in Ukraine.

The president and the Chancellor met days after both the United States and the European Union imposed new sanctions on key political and business figures around President Vladimir Putin.

The measures followed the failure of a deal brokered in Geneva to de-escalate tensions in Ukraine and the refusal of Russia to rein in pro-Moscow separatist groups in southeastern Ukraine.

So far, the sanctions adopted by either side have been limited to personal visa and asset bans against prominent people in Putin's inner circle, branded "cronies" by the White House.

But Washington warns that it will impose tougher sanctions that will hit directly at key sectors of the Russian economy, if Mr Putin for instance marches troops currently massed on the border of Ukraine directly into the country.

Such a scenario would entail a tough political choice for European leaders like Dr Merkel, who are under intense pressure from powerful business interests dismayed at the potential loss of important markets and investments in Russia.

On Thursday, White House spokesman Jay Carney denied Washington and the EU had butted heads over possible new sanctions.

"There has been a great deal of collaboration and cooperation in that effort between the United States and the EU, as well as all the members of the G7, so we expect that effort to continue," he said.

"We expect to continue a path that sees an international coalition escalating the costs that Russia will have to endure and pay if Russia refuses to keep its commitments."

Mr Carney added that the United States and EU members each have "a different kind of economic relationship with Russia, and so sanctions will affect different nations differently."

After their talks Dr Merkel and Mr Obama will hold a press conference in the Rose Garden of the White House, the site of a sumptuous state dinner hosted by the US leader for his guest three years ago.

Since then, relations between Berlin and Washington have been hit by the continuing damaging fallout over revelations of National Security Agency (NSA) spying in Germany, including the tapping of Dr Merkel's mobile phone, which the US says has now been stopped.

Mr Obama has sought to mend fences with Dr Merkel after the revelations by fugitive intelligence contractor Edward Snowden.

Mr Obama said in an interview broadcast in Germany in January that Dr Merkel had no need to worry about the United States spying on her in future.

But after Mr Obama extended the invitation for a visit, Dr Merkel noted that it would take "more than one trip" to repair the damage.

Her spokesman Steffen Seibert has said that, although Berlin last year pressed for a mutual "no-spy" pact with Washington, "concrete results" were not expected during Merkel's brief stay.

The United States never seemed willing to sign on to such an agreement, observers say, while Mr Obama stressed that his country would continue to conduct intelligence operations.

The two leaders will also try to give new momentum to the proposed EU-US free trade deal known as the Transatlantic Trade and Investment Partnership (Ttip) Dr Merkel supports the treaty in seeking opportunities for German companies, but there are doubts on both sides of the Atlantic whether the pact is politically viable.

During her trip to Washington, Dr Merkel will also address the US Chamber of Commerce, and meet International Monetary Fund chief Christine Lagarde, and US senators.

As Dr Merkel met Mr Obama, German lawmakers fumed over criticism by top Republican Senator John McCain's stinging criticism of her over Ukraine, saying she was blocking the adoption of tougher sanctions.

"I would tell her that I am not surprised but embarrassed at their failure of leadership. They're the leaders, they're being governed by the industrial complex from Germany," Mr McCain said.

Top-selling German newspaper Bild said Mr McCain had "attacked" Dr Merkel with "crass" comments.

A leading lawmaker from Dr Merkel's conservative Christian Democratic Union, Mr Karl-Georg Wellmann, told news website Spiegel Online that Mr McCain's accusation that industry called the shots for the German government was "vicious nonsense."