Despite losing troops, ground vehicles and aircraft in the fighting, Russia’s invading forces remain roughly 95% intact, a senior U.S. defense official said at a press briefing Tuesday.

Ukrainian resistance efforts are continuing to slow down Russian forces, particularly in the north, where they have not moved any closer to Kyiv’s city center since yesterday, the official said. Fighting and resistance against the Russian advance on Chernihiv and Kharkiv have been reported as well, the official said.

Russian air and missile attacks are increasing as the ground effort has been frustrated, and bombardment of the capital continues, he added. Much of the airspace of Ukraine, in the north and south, is under some umbrella of “Russian surface-to-air missile capability,” the official said.

"We still have every reason to assess that their effort is to encircle and force the surrender of Kyiv,” the official said.

In the south, Russians advancing to the northwest out of Crimea are about 25 miles out of Mykolayiv, the official said. Russian forces have isolated the city of Mariupol and continue to bombard it, he added.

In addition, Ukraine’s military is struggling to overcome shortages of fuel and food, he said.

The deconfliction line set up last Tuesday about has been used by the U.S. and Russia about a dozen times now, but so far only for test calls to make sure "somebody's picking up on the other end."

-ABC News' Mattew Seyler

The safety of the hundreds of staff who are still on duty at the Chernobyl Nuclear Power Plant is becoming more precarious, according to multiple agencies.

About 210 personnel have on duty since Feb. 24, the day before Russian military forces entered the disaster site, and have not rotated out, according to the agency.

Nuclear material and facilities demand continuous coverage, which requires employees to operate on a rotation, fixed, or modified shifts, according to a publication from the U.S. Department of Energy. There are many psychological and physiological impacts that can affect work performance, safety, and security without an organization, the material states.

Today, the State Nuclear Regulatory Inspectorate of Ukraine told the IAEA that it is becoming increasingly urgent and important for the safe management of the site to replace the current personnel.

While the staff has access to food, water and medicine to a limited extent, the situation is worsening, Ukraine’s nuclear regulator told the IAEA.

"I'm deeply concerned about the difficult and stressful situation facing staff at the Chornobyl nuclear power plant and the potential risks this entails for nuclear safety. I call on the forces in effective control of the site to urgently facilitate the safe rotation of personnel there," Rafael Mariano Grossi, Director General of the IAEA, said in a statement Tuesday.

Ukraine's nuclear regulator also asked the IAEA to lead the international support needed to prepare a plan for replacing the current team, which will include pausing the handling of nuclear material at the site, which includes decommissioned reactors as well as radioactive waste facilities

-ABC News' Christine Theodorou

Starbucks announced Tuesday it will immediately suspend all its operations in Russia, condemning the "horrific attacks on Ukraine by Russia."

"We continue to watch the tragic events unfold and, today, we have decided to suspend all business activity in Russia, including shipment of all Starbucks products," the company's chief executive officer, Kevin Johnson, said in a statement.

The company said its licensed partner agreed to immediately pause store operations and provide support for its nearly 2,000 workers.

"Through this dynamic situation, we will continue to make decisions that are true to our mission and values and communicate with transparency," Johnson said.

-ABC News' William Gretsky

McDonald's announced Tuesday it is temporarily closing its restaurants and pausing operations in Russia, as a result of the invasion of Ukraine.

"The conflict in Ukraine and the humanitarian crisis in Europe has caused unspeakable suffering to innocent people. As a System, we join the world in condemning aggression and violence and praying for peace," the company's chief executive officer, Chris Kempczinski, said in a statement.

McDonald's employs 62,000 people in Russia, operating in 850 communities, the company said. It will continue paying salaries for all its employees in Russia.

"Our values mean we cannot ignore the needless human suffering unfolding in Ukraine," said Kempczinski.

Kempczinski said it is impossible to predict when the company will be reopening its restaurants.

"We are experiencing disruptions to our supply chain along with other operational impacts. We will also closely monitor the humanitarian situation," Kempczinski said.

-ABC News' William Gretsky

The Russian forces charging toward Kyiv haven't made progress in the last day as they face Ukrainian resistance and low food and gas supply, a senior U.S. defense official told reporters Tuesday.

However, it could be a deliberate pause, the official said. "Part of the stall could be ... as a result of their own self-determined sort of pause in operations -- that they are possibly regrouping, rethinking, reevaluating," the official said.

The U.S. believes Russian forces "have committed now more than 80% of what was their pre-staged combat power," the official added.

The official said some Russian soldiers weren't told they were going into combat. The official said "not all of them were apparently fully trained and prepared."

The strong Ukrainian resistance has also hurt morale, according to the official.

Russia has now launched more than 400 missiles on Ukraine, the official said. The U.S. believes Russia has launchers that could be used for thermobaric weapons, but cannot confirm their use, the official said.

Russian forces are making the most progress in the south. Russians are attacking Kherson in south Ukraine, which "appears very much to be contested city at this point," the official said.

Russians are also approaching Mariupol in southeastern Ukraine, and while they haven't yet entered the city, "they are close enough now that they could attack Mariupol with long range fires," the official said.

Two towns on the path to Mariupol are believed to be occupied by the Russians, according to the official.

The U.S. believes the Russians hope to move north out of Mariupol up to the heavily-contested city of Kharkiv. The official said they believe Russian forces are trying to encircle Kharkiv.

The U.S. official noted that they've seen "certain risk-averse behavior by the Russian military" over the last week.

"Take the amphibious assault, for instance. They put those troops ashore a good 70 kilometers away from Mariupol because they knew Mariupol was going to be defended and they could put them ashore in an uncontested environment. And they still haven't reached Mariupol," the official said.

"They are not necessarily willing to take high risks with their own aircraft and their own pilots," the official said.

"And of course we're seeing that on the ground -- the fairly slow and steady progress that they have made, and you guys are seeing it for yourselves on the ground where ... units are surrendering, sometimes without a fight."

-ABC News' Matt Seyler