The US conceded on Sunday that Moscow had “complete operational control of the Crimean peninsula” and announced that the secretary of state, John Kerry, will fly to Kiev in an attempt to halt a further Russian advance into Ukraine.

Senior US officials dismissed claims that Washington is incapable of exerting influence on the Russian president, Vladimir Putin, but were forced to admit that Crimea had been successfully invaded by 6,000 airborne and ground troops in what could be the start of a wider invasion.

“They are flying in reinforcements and they are settling in,” one senior official said. Another senior official said: “Russian forces now have complete operational control of the Crimean peninsula.”

On Monday, Ukrainian border guards reported a buildup of armoured vehicles near a ferry port on the Russian side of the Kerch Channel – a narrow sea channel dividing Russia and the Ukrainian. A statement from the guard spokesperson said Russian ships had also been moving in and around the city of Sevastopol, where the Russian Black Sea fleet has a base, and that Russian forces had blocked telephone services in some areas.

Although President Barack Obama’s administration called for Putin to withdraw troops to Russian military bases on the peninsula, its objective appeared to have shifted to using political and economic threats to prevent any further military incursion.

One senior official said the major decision facing Putin was whether to “continue to escalate troop movements into other parts of Ukraine”.

“We’ve already seen the intervention in Crimea,” the official said. “It would be even further destabilising to expand that intervention into eastern Ukraine.”

The official added: “Our bottom line is they had to pull back from what they’ve already done, go back into their bases in Crimea. We’ll be watching very, very carefully of course and will be very, very concerned if we saw further escalation into eastern Ukraine.”

Kerry will fly to Kiev on Tuesday, to meet Ukraine’s new government and display “strong support for Ukrainian sovereignty”, a state department official said. However, in Washington there were mounting questions, particularly from Republican opponents of the administration, about the influence Kerry and other officials have over Moscow.

Kerry, Obama and other senior officials spent the last 24 hours frantically attempting to rally an international coalition of countries to condemn Moscow over the Crimea invasion, and commit to economic sanctions in order to prevent a further advance into other pro-Russian parts of Ukraine.

Obama spoke by phone with the British prime minister, David Cameron, Polish president Bronisław Komorowski and the German chancellor, Angela Merkel.

“We are concerned as we watch this situation that the Russians have badly miscalculated,” one of the senior officials said. “There is a very fierce and proud tradition in Ukraine of defending their sovereignty and territorial integrity. So far Ukraine has showed, and Ukrainians individually have showed, marked restraint … but the longer this situation goes on, the more delicate it becomes.”

Earlier on Sunday, Kerry told CBS leading western nations were prepared to enact economic sanctions against Russia over what he called an “incredible act of aggression”.

“You just don’t in the 21st century behave in 19th-century fashion by invading another country on completely trumped up pretext,” Kerry said. “It is really a stunning, wilful choice by President Putin to invade another country. Russia is in violation of the sovereignty of Ukraine. Russia is in violation of its international obligations.”

Asked how the US and its allies might respond, Kerry stressed the economic harm that could befall Russia if it continued its occupation of Crimea, but repeatedly said “all options” were under consideration.

However, in a conference call with reporters later on Sunday, three senior US administration officials made clear that the “menu” of options before the White House does not include military action.

“Frankly, our goal is to uphold the territorial integrity and sovereignty of Ukraine, not to have a military escalation,” one of the officials said. “I don’t think we’re focused right now on the notion of some US military intervention. I don’t think that would be an effective way to de-escalate the situation.”

During the call, which last almost an hour, the officials said they were looking to provide Russia with “off-ramps” that would enable Putin to reverse his course, and were applying pressure through a broad international coalition that had agreed to to ostracise Moscow.

That process has begun with major powers pulling out of preparatory meetings ahead of the G8 summit which is due to be hosted in Sochi in June, as well as the cancellation of other trade-related meetings with Russia planned for this week. In effect, Russia is being threatened with expulsion from the G8 group of countries, unless it withdraws from Ukraine.

That will quickly escalate to possible sanctions, including potential visa and banking restrictions targeting Russians close to Putin. Currently, the US is reviewing “all of our economic and trade cooperation with the Russian Federation”, one official said, and all 28 members of Nato were planning to sign up to a single statement, strongly condemning Moscow.

“He [Putin] is not going to have a Sochi G8, he may not even remain in the G8 if this continues,” Kerry told NBC earlier in the day. “He may find himself with asset freezes, on Russian business, American business may pull back, there may be a further tumble of the ruble.”

The Obama administration is also working with the European Union and International Monetary Fund to fast-track a package of financial aid and loans, in order to shore-up Ukraine’s economy.

The officials argued that Russia had miscalculated by invading Ukraine and effectively conquering the Crimean peninsula. What US officials described as the Russian “intervention” was likely to bolster “the people of Ukraine’s desire to reorient towards Europe”, an official said.

Another senior official said: “When it comes to soft power, the power of attraction, Vladimir Putin has no game. So he’s left with hard power and it’s a very dangerous game to play.”

However, the senior officials sounded flustered as they struggled with accusations from reporters that Obama had shown himself to be powerless in the face of Russian aggression.

On Friday, Obama made a forceful public address, warning Putin that there would be “costs” if Russia intervened in Ukraine. On Saturday he spent 90 minutes on the phone with the Russian leader, ultimately failing to dissuade him from taking military action.

Asked if Obama had a “credibility problem”, one senior official replied: “The premise of your question is he [Putin] is strong and [the] president of the United States is weak. He [Putin] is not acting from a position of strength right now.”

The official added: “You’re seeing the ability of the United States to bring with us … the rest of the G7 countries, the rest of Nato, and frankly the large majority of the world in condemning this action.”