Updated at 4:47 p.m.

President Obama blamed Russia for the violence in Ukraine and said its "incursion" into the former Soviet state will only carry additional costs.

"Russia has deliberately and repeatedly violated the sovereignty and territorial integrity of Ukraine, and the new images of Russian forces inside Ukraine make that plain for the world to see," Obama said at a White House news conference on Thursday.

Maxim Zmeyev/Reuters /Landov

Both pro-Russian rebels and the Ukrainian government forces are denying any responsibility for downing the Malaysia Airlines jetliner carrying 295 passengers and crew. Multiple reports state the Boeing 777, traveling from Amsterdam to Kuala Lumpur, appeared to have broken up before impact, with the burning wreckage scattered over a wide area. All passengers are believed to have died. 

At a press event in Delaware, President Obama said he has directed national security advisors to stay close contact with the Ukrainian government. Reuters reports that just yesterday, the U.S. administration tightened its sanctions against Russia over the ongoing fighting in Ukraine. Shortly thereafter, Ukraine claimed that Russian had downed one of its war planes.

U.S. troops began joint military exercises in Poland this week, in response to Russia's continued incursion into Ukraine.  The Pentagon says troops will also be deployed to Lithuania, Latvia and Estonia.

Vice President Joe Biden arrived in Kiev on Monday as a show of support for the pro-Western Ukrainian government. As he met with the nation's leaders he called on Russia to "stop talking and start acting" to defuse the crisis.

So far, that doesn't seem to be happening. 

The Russian troops who are holding Crimea won't be sent into Ukraine, Russian Foreign Minister Sergei Lavrov says. "We have absolutely no intention of — or interest in — crossing Ukraine's borders," Lavrov told a Russian TV station Saturday, according to a translation by Reuters.

There will be few days that better symbolize the crisis in Ukraine.

On Friday:

As Ukrainian Prime Minister Arseniy Yatsenyuk was signing an agreement on closer relations with the European Union ...

... Russian President Vladimir Putin was signing the laws his country has put in place to take Crimea from Ukraine and make it part of the Russian Federation.

This post was updated at 4:50 p.m. ET.

Russia's parliament has unanimously approved a request by President Vladimir Putin to authorize the intervention of Moscow's forces in Ukraine until "the normalization of the political situation" there. In response, Ukraine put its own forces on alert and warned that a Russian invasion would spark war between the two countries.