Ukraine's new interior minister says Russian naval soldiers are blocking Crimea's Sevastopol airport, while other Russian forces are patrolling the airport in the regional capital, Simferopol.
Arsen Avakov described the airport seizures as "a military invasion and occupation."
Interfax quoted military sources in the region as saying that the soldiers had gone to Sevastopol's airport to stop "militants" from flying in.
Ukraine's UNIAN news agency said six military trucks full of armed soldiers had arrived at the airport, which also doubles as a Ukrainian Air Force facility.
Russia's Black Sea Fleet, which has a base in Sevastopol, denied its forces were involved in seizing or blockading the airport.
Ukraine's parliament, meanwhile, has urged Moscow to stop any moves that could undermine the country's sovereignty.
The Verkhovna Rada called on the United States, Britain, and Russia to uphold Ukraine's territorial integrity in line with a memorandum they signed in 1994
. It also suggested the UN Security Council meet to consider the crisis.
Also, acting President Oleksandr Turchynov has dismissed the chief of staff of Ukraine's armed forces. A decree on the dismissal of Yuriy Ilyin as chief of the General Staff and head of the armed forces was posted on Turchynov's official website on February 28.
Ilyin was promoted to chief of the General Staff by President Viktor Yanukovych during the deadly unrest in Kyiv earlier this month that ultimately resulted in Yanukovych's ouster.
Turchynov, meanwhile, has called an emergency meeting with his security chiefs later on February 28 to discuss the latest developments in Ukraine's Crimea region.
U.S. Secretary of State John Kerry on February 27 called on all sides to "avoid provocations." He said he had received assurances from his Russian counterpart, Sergei Lavrov, that Russia would respect Ukraine's territorial integrity.
Tensions in Crimea, Ukraine's only majority ethnic Russian region, have been boiling following the ouster of pro-Moscow Yanukovych.
On February 27, masked gunmen seized the regional parliament in Simferopol and the government building and raised the Russian flag. The Crimean parliament later announced it will hold a referendum on May 25 on expanding Crimea's autonomy.
A man who identified himself as a volunteer helping the armed men who took control of Simferopol airport earlier on February 28 said the move was aimed to prevent "radicals" from Kyiv coming in.
"It was our initiative in order to stop the fire from spreading into Crimea. That's why we don't want radicalism to come here," he said. "We don't want fascism in Crimea. We want peace in Crimea."
WATCH: Armed troops occupy Simferopol's airport.
Crimean lawmakers on February 27 also dismissed the existing government and appointed a new prime minister. Serhiy Aksyonov said on February 28 he still considered Yanukovych to be Ukraine's legitimately elected president.
Meanwhile, Yanukovych has give his first news conference since he was ousted on February 22 in the southern Russian city of Rostov-on-Don.
He said he he intends to "continue the struggle for the future of Ukraine" denounced the new Ukrainian authorities as "fascist thugs" and said they represent only a minority of Ukrainians. He added that the country's parliament, the Verkhovna Rada, is illegitimate and making decisions under the threat of armed force.
In the past week, parliament has overthrown Yanukovych's authority and appointed a new interim president.
Ukraine's new authorities have also issued an arrest warrant for Yanukovych on charges of "mass murder" over the killings of dozens of antigovernment protesters during Ukraine's recent unrest. Ukraine's prosecutor has also said Kyiv will ask Russia to extradite Yanukovych if it is confirmed he is in Russia.
New Prime Minister, Cabinet
Arseniy Yatsenyuk, one of the leaders of the anti-Yanukovych movement, was confirmed on February 27 as prime minister of a new government led by the former opposition.
U.S. Vice President Joe Biden called Yatsenyuk afterward and pledged Washington's support for his government. Biden added that the United States welcomed the formation of Yatsenyuk's government.
Yatsenyuk said it was unknown how loans worth $37 billion had been used during Yanukovych's term, and that $70 billion was transferred from Ukraine's financial system to offshore accounts.
The International Monetary Fund (IMF) has announced it's sending a team to Ukraine for urgent talks on a possible bailout worth billions of dollars. The team is expected to visit Kyiv next week.
Ukraine's new authorities have estimated that more than $15 billion in IMF loans may be needed. In return, the IMF is expected to require reforms that could have a painful impact on the economic conditions of ordinary people.
In related news, the central bank has imposed limits on foreign-currency withdrawals as Ukraine's currency plunges to record lows.
National Bank Governor Stepan Kubiv said on February 28 that withdrawals will be limited to around $1,500 per day.
The hryvnya has plunged nearly 20 percent since the start of the year amid Ukraine's political crisis, and foreign-currency reserves dropped to $15 billion from $17.8 billion early this month.
Yatsenyuk said on February 28 that Ukraine hopes to begin receiving international financial aid soon and is determined to fulfill conditions needed to secure support from the IMF.
The IMF's involvement comes after Russia raised doubts about the future of a $15 billion aid package to Kyiv that Moscow agreed with Yanukovych's government in December.
On February 28, Russian President Vladimir Putin issued a series of orders to the government regarding relations with the new Ukrainian government.
In a statement posted to the Kremlin website, Putin instructed Russia's government to continue economic and trade talks with Kyiv while also cooperating with the IMF and G8 countries on financial aid for the Ukrainian government.
Presidential spokesman Dmitry Peskov also told Interfax that Putin had also told the government to consider "the Crimean authorities' request for humanitarian aid."
'Robbed And Empty'
Speaking to the Verkhovna Rada national assembly on February 27, Yatsenyuk said Ukraine's treasury "has been robbed and is empty."
He warned that unpopular measures will be necessary to stave off economic collapse. But he vowed steps to move the country toward European Union integration.
Yanukovych's refusal to sign accords on cooperation with the EU in November sparked the protest movement that has resulted in the new government.
"The key task for the new Ukrainian government is European integration," Yatsenyuk said. "It means visa-free regime for the Ukrainian citizens and it means an agreement with the European Union on political and economic integration, agreement on fully fledged free-trade zone. The future of Ukraine is in Europe and Ukraine will become a member of the European Union."
In related news, a Swiss prosecutor has launched a criminal investigation into alleged money laundering by Yanukovych and his son Oleksandr. The prosecutor's office said on February 28 that the focus of the probe was "aggravated money laundering."
The prosecutor, Yves Bertossa, and the police searched a company owned by Oleksandr Yanukovych and seized some documents.
The Swiss government on February 28 announced it was freezing the assets of 20 Ukrainians, including Yanukovych and his son.
Also on February 28, the Austrian government said it had frozen the assets of 18 Ukrainians suspected of violating human rights and of involvement in corruption. It did not release the names of the 18 individuals.
The EU last week agreed on a travel ban and asset freeze on Ukrainians with "blood on their hands," but did not name those affected. It said the number would depend on developments.
With reporting by Interfax, UNIAN, Reuters, AFP, AP, and Al-Jazeera