BRUSSELS — The European Commission said on Thursday that airlines that did not follow a new European law requiring them to account for their emissions of greenhouse gases could face being banned from European airports.

The warning was the latest stage in an escalating war of words between the European Union and countries like China, which have expressed fierce opposition to a law that represents the European Union’s boldest move to date to protect the climate.

The initiative went into effect at the start of the year and involves folding aviation into the European Union’s six-year-old Emissions Trading System, in which polluters can buy and sell a limited quantity of permits, each representing a ton of carbon dioxide.

A European ban on noncompliant airlines would be a measure of “very last resort” applicable only in cases of “continued noncompliance,” Isaac Valero-Ladron, the commission’s spokesman for climate action, said on Thursday at a news conference in Brussels.