Thread by @DanLamothe

Sunday background briefing with senior U.S. defense official winding down.

Up front, official acknowledges reports that Putin has put his nuclear forces on alert and said that it is “not only an unnecessary step for him to take, but an escalatory one.”

Senior U.S. defense official says that Russia continues to face stiff resistance on its advances. Also says Russia is struggling with fuel and logistical issues. Still no indications that Russia has taken control of any major cities, U.S. official says.

Significant: Russia has now adopted siege tactics on the Ukrainian city of Chernihiv, senior U.S. defense official says. That significantly elevates the likelihood of civilian casualties there.

As noted yesterday, Russian reconnaissance elements have been spotted in Kyiv. Senior U.S. defense official goes farther today and says some have been discovered in Ukrainian military uniforms, either by local people or Ukrainian forces.

Russia is about 30 kilometers from Kyiv’s city center and about 50 kilometers from Mariupol city center, U.S. defense official says. Belief is that the forces west of Mariupol include both ground forces coming from Crimea and naval forces who made an amphibious landing.

Pentagon assesses that airspace over Ukraine remains contested. Ukrainian aircraft and air defense systems still up, though degraded, senior U.S. defense official says.

Russia now has about two-thirds of its combat power initially arrayed at the Ukrainian border in the fight, senior U.S. defense official says. That’s up from about 50 percent in Saturday’s assessment.

Russia has now launched 320 missiles at Ukraine, up from 250 in Saturday assessment from the Pentagon. There have been indications that some launches have “experienced failures.”

Senior U.S. defense official says there are no indications of blockages of arms flowing to Ukrainians from the west. Official says he doesn’t want to go farther to make sure the arms get into the right hands.

Senior U.S. defense official warns that we are still in Day 4 of an operation with no end in sight. Hard to predict how it will go, given the bloody, unpredictable nature of ground combat.

“Everybody needs to look at this with a sense of humility here," official says.

Senior U.S. defense official: "We are in Day 4. The Russians will learn and will adapt and will try to overcome these challenges. I think we need to be pragmatic about that."