President Obama announced Thursday that the U.S. was expanding sanctions against senior Russian officials following Moscow's annexation of Crimea and warned of further “severe actions” against the Kremlin if it did not stand down.

“The world is watching with grave concern,” Obama said from the South Lawn of the White House, announcing that he signed an executive order on Thursday giving him the authority to “impose sanctions not just on individuals but key sectors of the Russian economy.”

The first round of sanctions from the White House against Russia were shrugged off by Russian President Vladimir Putin, raising questions about whether Obama had an effective blueprint to deter the Kremlin's take over of Crimea.

Obama said Thursday that in addition to expanding the list of those being sanctioned, his administration also penalized an individual bank that conducts business with the officials in question.

Detailing the additional punishments, senior administration officials said 20 senior Russian government officials and “cronies” received travel bans and asset freezes.

Bank Rossiya, the 17th largest bank in Russia — and described by one senior administration official as “Putin’s personal banker" — also had its U.S. assets frozen.

Though Obama didn’t pull the trigger on sanctions against sectors of the Russian economy, the new executive authority gives him the ability to do so in the future.

Obama now has the capability to target Russia’s financial services, energy, metals and mining, defense and engineering sectors, the official explained.

“Nobody should believe this is the end of what we’re prepared to do under the executive order,” a senior administration official said. “It's only the beginning.”

In response, the Kremlin issued retaliatory sanctions against U.S. officials, including Speaker John Boehner, R-Ohio, Senate Majority Leader Harry Reid, D-Nev., Sen. Robert Menendez, D-N.J., Sen. John McCain, R-Ariz., Sen. Mary Landrieu, D-La., and White House senior adviser Dan Pfeiffer.

“The speaker is proud to be included on a list of those willing to stand against Putin’s aggression,” said Boehner spokesman Michael Steel.

The expanded U.S. sanctions came after the administration earlier this week froze assets and issued travel bans against 11 senior Russian and Ukrainian officials.

Analysts, however, say that anything short of sanctions against Russian banks and energy companies won’t command Putin’s attention.

In the wake of the Crimean referendum to join Russia, the situation on the ground has become increasingly volatile. Ukrainian troops were forced to flee their bases Wednesday and government officials have sounded the alarm over fears of a full-blown Russian invasion.

Putin has claimed to have little interest in taking over any part of Ukraine beyond Crimea but also added that the Kremlin would act to preserve Russian interests across the region.

Pro-Russian forces on Thursday took over a pair of Ukrainian warships, just a day after storming the navy headquarters in the Crimean port of Sevastopol.

European leaders are also crafting additional sanctions against Russia. German Chancellor Angela Merkel said Thursday the European Union would expand the travel bans and freezing of assets and prepare to move to more wide-ranging economic penalties against Russia.

Obama on Wednesday ruled out the possibility of military intervention in Ukraine, a prospect dismissed by Democrats and Republicans alike in Washington.

However, conservatives argue that the administration should step up military aid to Ukraine and work with partners to shore up the Eastern European nation’s economy.

Amid the turmoil, however, Obama insisted that Putin still has an exit ramp from the crisis.

“Diplomacy between the United States and Russia continues,” he said. “The Ukrainians should not have to choose between the West and Russia.”

This story was published at 11:21 a.m. and has been updated.