WASHINGTON, Feb 27 (Reuters) - Russia, frustrated by early battlefield setbacks in Ukraine, could be shifting its strategy to siege warfare just as President Vladimir Putin raises the risk of a catastrophic miscalculation by putting nuclear forces on heightened alert, a senior U.S. defense official said on Sunday.

Citing a Russian offensive on the Ukrainian city of Chernihiv, north of Kyiv, the official cited early indications that Russia might be adopting siege tactics.

"It appears that they are adopting a siege mentality, which any student of military tactics and strategy will tell you, when you adopt siege tactics, it increases the likelihood of collateral damage," the official said.

The senior U.S. defense official said it remained to be seen what Russian forces would do next, but the early signs were worrying.

"The indications are enough in terms of how they're positioning their forces around the city (of Chernihiv), how they're beginning this barrage using rockets, that gives us concern," the official said.

现在是2月底，城外的土地全是烂泥巴和沼泽........俄爹又一次展现了强大的战略智慧。