MOSCOW (Reuters) - Russia said on Friday it would respond quickly with retaliatory measures to the latest round of U.S. sanctions that it criticized as another "hostile step", pledging to support affected companies with state funds.

The European Union and the United States have tightened economic penalties on Moscow, accusing it of sending troops to back pro-Moscow separatists fighting Ukrainian government forces in eastern regions.

"Of course our retaliatory measures will not keep you waiting," the Foreign Ministry said in an online statement.

"We see Washington's introduction of new anti-Russian sanctions as another hostile step in line with the confrontational course taken by the American administration."

The ministry said the sanctions were short-sighted and would have no effect on government policy.

Earlier on Friday President Vladimir Putin, who has regularly denied any involvement in Ukraine and already responded with import bans and other steps, said any retaliatory measures would be taken carefully to avoid damaging the Russian economy.

The Kremlin has pledged to support companies affected and on Friday Economy Minister Alexei Ulyukayev said Russia may divert funds from its National Wealth Fund or from pensions.

"There are different forms of support. This includes various custom tariff regimes ... possibly direct budget support (and) the possibility of using pension funds or the National Wealth Fund and other mechanisms," he told journalists in Brussels in a statement later published on the Economy Ministry website.

Spending pressure on the government has increased as a result of the Ukraine crisis, which has led several Russian companies to request state finance to compensate for the closure of Western capital markets.

Last week, Putin said Russia cannot spend more of the $83 billion parked in its National Wealth Fund after the government raised the cap on how much of the fund can be used for domestic investments.

In August, Russia's largest oil company Rosneft asked the government for a $40 billion cash injection, to be financed from the NWF, to help it weather the sanctions.

Ulyukayev also said Russia would appeal to the World Trade Organisation. "The latest round of sanctions gives reason to appeal to the WTO. And we will appeal," RIA news reported him as saying in Brussels.

The drive for tougher sanctions has faced growing opposition from a number of EU countries that fear retaliation from Russia, the bloc's biggest energy supplier.

The EU has said it could lift some or even all of the sanctions if Moscow abides by a fragile truce in Ukraine and other parts of a peace plan agreed to try to end the worst confrontation between Russia and the West since the Cold War.

(Reporting by Alessandra Prentice; Editing by Alison Williams)