Alexander Vershbow is a distinguished fellow at the Atlantic Council’s Scowcroft Center for Strategy and Security. He was previously deputy secretary general of NATO, U.S. ambassador to Russia and assistant secretary of defense.

Russia’s illegal blockade and closure of the Kerch Strait on Sunday and its violent seizure of three Ukrainian ships are clear violations of international law and of bilateral Ukrainian-Russian agreements. Unlike the Russians’ 2014 illegal annexation of Crimea and their undeclared war in Eastern Ukraine, this was not an ambiguous, deniable attack by “little green men” without insignias; it was a direct act of aggression by Russia’s navy and security services against a sovereign state that was acting within its rights in the Black Sea and the Sea of Azov, whose waters are shared by Russia and Ukraine. These actions can only be seen as premeditated actions aimed at destabilizing Ukraine in the run-up to Ukraine’s 2019 elections and crippling its economy.

The Trump administration has condemned these steps rhetorically but has yet to threaten any specific measures in response. That is a mistake. If Sunday’s events bring little more than a slap on the wrist, Russian President Vladimir Putin will see this as a green light for further escalation, to include a full blockade or even the illegal annexation of the Azov Sea, as well as new offensive actions in Eastern Ukraine.

It’s important to remember that Sunday’s attacks were not a one-off event. Russia has been intensifying the pressure on Ukraine over the past three to four months in small steps, similar to what we saw in Moscow’s creeping aggression against Georgia in the spring and summer of 2008. This may be based on the hope that each small step will be met with nothing more than political protests by the West.

Other Russian actions look ominous in retrospect. In recent months, Moscow has launched incessant artillery and rocket attacks by Russian-led forces on Ukrainian military and civilian targets along the line of contact with occupied Donbas. The Russians have interfered with international shipping to and from Ukraine’s Azov Sea ports of Mariupol and Berdyansk throughout the summer, inflicting significant losses on the local and national economy. The Kremlin imposed sweeping sanctions on Ukrainian companies and pro-Western business leaders earlier this month. The Russians also allowed the holding of elections in occupied portions of the Donetsk and Luhansk districts in flagrant violation of the 2015 Minsk agreements.

Moscow seems determined to show that Ukraine is a failed state that cannot defend its own borders in the hope of bringing more malleable leaders to power in Kiev in next year’s elections and reversing Ukraine’s rapprochement with the European Union and NATO. Ukraine’s sovereignty and its aspirations for a European future are, therefore, very much on the line, as is the credibility of the West’s commitment to an international order based on the rule of law rather than the law of the jungle.

Rhetorical condemnation by the United States and the European Union is necessary but not sufficient. The United States and its European allies need to impose real costs on Russia if it doesn’t reverse course, including tighter economic sanctions on Russian shipping companies, banks and individual Putin cronies involved in trade with illegally occupied portions of Ukraine.

An essential first step should be seeking agreement by Germany to freeze or, preferably, cancel the Nord Stream 2 gas pipeline from Russia to Germany via the Baltic Sea. Once operational, Nord Stream 2 — which bypasses Ukraine — will make the Ukrainian gas transit system unsustainable, costing Ukraine billions of dollars in lost revenue and a 3 percent drop in GDP. Russia’s multiple provocations make absurd European efforts to obtain Russian guarantees of continued gas transit through Ukraine after Nord Stream 2 comes on line. If Berlin balks, the United States should extend sanctions under the Countering America’s Adversaries Through Sanctions Act to international energy companies participating in the Nord Stream 2 project.

The United States and its allies should also expand NATO’s naval presence in the Black Sea to demonstrate support for Ukraine and for freedom of navigation in international waters, which has been challenged by Russia’s actions. The United States and its allies should consider increased support to the Ukrainian Navy as well, including provision of coastal defense systems for deployment along the Azov Sea coast. Without such measures, Russia will believe it can continue to escalate with impunity — today against Ukraine, tomorrow against the West.

The United States should take action before President Trump meets with Putin on the margins of the Group of 20 meeting in Argentina later this week. Improving U.S.-Russian relations will be possible only if Putin is faced with American strength and resolve, not empty rhetoric. Trump has promised to be tougher on Russia than his predecessor, who allegedly “allowed Russia to take Crimea,” according to Trump. The question today is: Will Trump let Russia take the Azov Sea as well?